Activated carbon - A filter medium used to remove oil, water and odors from breathing air.

Airlift (dredging device) - A device based on a pipe, used in by divers to suck small objects, sand and mud from the sea bed and to transport the resulting debris upwards and away from its source. Air is injected into the lower end of the pipe and the rising bubbles entrain water and cause an upward flow which draws the material from the bottom along.

Algal bloom - A rapid increase or accumulation in the population of algae (typically microscopic) in an aquatic system. Some blooms may be recognized by discoloration of the water resulting from the high density of pigmented cells. Visibility can be severely impaired over a period of hours to days.

Algorithm - A set of equations incorporated into diving computers in order to compute nitrogen uptake and elimination from changes in depth and elapsed time.

Alternative air source - A secondary supply of air or other breathing gas used by the diver in an emergency

Alternobaric vertigo - Dizziness caused by a difference of pressure between the middle ears.

Altitude diving - Diving at a location where the water surface is at an altitude which requires modification of decompression schedules. more than about 300 m (980 ft above sea level.

Aluminum alloy 6061 - (not subject to sustained load cracking). Used for manufacture of new diving cylinders exclusively since mid 1988 as it is not susceptible to sustained load cracking.

Aluminum alloy 6351 - This alloy is subject to sustained load cracking and requires periodical eddy current testing. Not used for new cylinder manufacture since 1988, but many cylinders of this alloy are still in service.

Ambient pressure - Pressure of the surroundings

Arterial gas embolism - Blockage of an artery by a gas bubble. A possible consequence of lung overpressure injury.

Association of Diving Contractors International - A non-profit organization to promote commercial diving, and establish and encourage observance of safe standards for commercial diving

Ata or atmospheres absolute - Unit of absolute pressure equivalent to standard atmospheric pressure

Automatic Diluent Valve - A demand valve set into the breathing loop of a rebreather to inject diluent gas into the loop when the loop volume falls and there is not enough gas for inhalation

Backplate - A plate, normally made from metal, which rests against the diver’s back, and to which the primary scuba cylinders are attached. Held to the body by harness straps over the shoulders and round the waist. Sometimes also crotch straps and chest straps. Usually used with a back inflation buoyancy compensator. The backpack supports and stabilizes the scuba cylinder on the diver's back.

Backward kick - A fining technique for moving backwards. Not an easy, powerful or elegant kick, but useful in many situations. The fins are angled outwards in opposite directions with the legs straight, then swept upwards and towards the diver by bending the knees in the power stroke. The knees may move downwards a bit at the same time by bending at the hips for stability. The return stroke feathers the fins by pointing them backwards in line with the body axis, to reduce forward thrust until the legs are straight again.

Backward roll - Water entry method in which the seated diver rolls backwards off the side of the boat, allowing the scuba cylinders to strike the water first

Bailout cylinder - A scuba cylinder carried by an underwater diver for use as an emergency supply of breathing gas in the event of a primary gas supply failure.

Balanced regulator - Regulator designed to provide a consistent demand effort not affected by cylinder gas pressure or depth

Barodontalgia - Tooth pain caused by pressure difference

Barotrauma - Injury caused by pressure difference.

Belt slider - Hardware item with two parallel slots which is fitted to harness or weight belt webbing to prevent other components such as D-rings and weights from sliding along the webbing

B-ring - A small plate with a slot for a webbing belt and two side by side holes for clipping on equipment, generally used similarly to a D-ring in combination with a belt slider as an alternative to a butterfly slider

Bent D-ring - A D-ring which has been bent about 45° near the straight section on both sides, forcing it to project slightly from the harness when pushed to one side, allowing easier attachment of clips.

Bends - Decompression sickness: Injury caused by bubble formation in the body tissues after hyperbaric exposure.

Billy ring - Three D rings welded together along their straight sides so that one is perpendicular to the other two. This uses the two flat rings to maintain the third in an upright position when mounted on harness webbing, allowing it to be more easily accessed to clip on stage cylinders. Named after Captain Billy Deans.

Blending stick - Mixing tube in which gases are continuously mixed prior to intake by a compressor, usually at atmospheric pressure. Usually refers to manufacture of nitrox from air with added oxygen, but also used for trimix. Gas mixture is usually continuously analyzed at the exit of the blending stick to monitor composition.

Block adapter - Screw-in adapter fitting which is fitted to a 200/240 bar DIN pillar valve to allow connection of a yoke regulator or filling whip.

Blue hole - A sink hole in a lake or the sea that is often the entrance to a cave. Blue holes in the sea are subject to tides so that their flows regularly reverse.

Blue-water diving - Underwater diving in mid-water where the bottom is not visible and is out of diving range.

Bolt snap - A metal connector comprising a hook with a spring-loaded axial sliding rod which must be manually retracted to allow the hook to be clipped onto something or removed. May be single- or double-ended, and if single-ended is usually fitted with a swivel ring opposite the jaws.

Booster or booster pump - Machine used to increase pressure of a gas. Usually refers to the case where inlet pressure is above ambient pressure already.

Bottom gas - The gas breathed by the diver at the deepest part of the dive. Compare with travel gas and decompression gas.

Bottom time -Time used in calculating decompression obligation from decompression tables. For most tables this is defined as the elapsed time from starting the descent to starting the final ascent to the surface, excluding ascent and decompression time.

Bottom timer - Device used to measure and record the total time spent underwater during a dive. They do not generally only record bottom time .

Bounce dive - a bounce dive is a descent to maximum depth and then a direct ascent back to the surface with the minimal bottom time, in a dive profile resembling a spike.

Bowline - Knot used to form a secure, non-slipping loop at the end of a line.

Boyle's law - Relationship between pressure and volume at constant temperature in an ideal gas.

Breathing loop - The gas flow path in a rebreather comprising the diver's lungs, the mouthpiece, valves, hoses, counter lungs and scrubber through which gas is rebreathed.

Bubble models - Decompression models based on the assumption that bubbles will form during non-symptomatic decompression.

Buddy bottle - Bailout cylinder carried by a scuba diver, particularly when diving solo.

Buddy breathing - Sharing breathing gas from one demand valve by two or more divers, generally after an out-of-gas emergency.

Buddy check - A procedure carried out by scuba divers using the buddy system where each diver checks that the other's diving equipment is configured, fitted, and functioning correctly just before entering the water to dive.

Buddy diving - A safety procedure where two or three divers monitor each other constantly during a dive and provide assistance or rescue when needed.

Buddy line - A short line between two divers, used to maintain contact during a dive, generally in poor visibility, or other conditions where the divers might become separated and not be able to quickly locate each other.

Buddy system - A procedure where two divers look out for the safety of each other, and give assistance if the other gets into difficulty.

Bühlmann algorithm - Diving tables and decompression algorithm on which the tables are based, and some dive computers are programmed, based on the dissolved gas decompression model derived and tested by Dr A.A. Bühlmann.

Buoyancy - Upward force on an object immersed in a fluid due to pressure exerted over the immersed surface.

Buoyancy check - Procedure to test and adjust weights carried by an underwater diver. The diver wears all the personal equipment to be used for the planned dive, with the scuba tank(s) nearly empty, and the buoyancy compensator empty, in shallow water of the same density as expected on the dive, and adds or removes weights until neutrally buoyant. After the buoyancy check it is usual to distribute the weights for safety, trim and convenience.

Buoyancy compensator - also BC, BCD, buoyancy compensator device, ABLJ, horse collar, stabilizer jacket, stab jacket or wing An airtight bladder worn by a diver which can be filled with air and vented to adjust and control the buoyancy of the diver.

Buoyancy control - The skill of maintaining the appropriate buoyancy at any time during a dive.

Burst disk - A non-reclosing pressure relief device used to protect a diving cylinder from over pressurization.

Butt-plate - A rigid or fairly stiff flexible lower extension to a backplate or other scuba harness supporting butt-plate rails, used for clipping off the lower end of sidemount cylinders to the harness.

Cage Diving - Diving in a cage designed to protect the diver from potentially aggressive large marine animals, usually sharks

Cam band - A strap, usually of webbing, with a cam action tensioner buckle generally used to secure a diving cylinder to a backplate, stabilizer jacket BCD or other form of diving harness.

Canister light - Dive light comprising a light head connected to a battery canister by a cable.

Canoe diving - Scuba diving from canoes, used when the dive site is beyond convenient swimming distances.

Carbon dioxide poisoning - The toxic effects of carbon dioxide, due to incomplete elimination of carbon dioxide resulting from skip breathing, excessive work of breathing, scrubber failure in a rebreather system, or inadequate ventilation in a diving chamber or free flow helmet. Occasionally caused by contaminated gas supply.

Carbon monoxide poisoning - The toxic effects of carbon monoxide, usually due to contaminated breathing gas supply.

Cardio-pulmonary resuscitation/CPR - An emergency procedure which is performed in an effort to manually preserve intact brain function until further measures are taken to restore spontaneous blood circulation and breathing in a person in cardiac arrest

CAS - Critical Air Supply, the amount of breathing air required to safely exit a penetration dive. When the air supply reaches this level the dive has reached a planned turning point.

Cascade filling - Several storage cylinders in succession, generally using a procedure to maximize charge pressures. Often used in partial pressure gas blending.

Caustic cocktail - A mixture of water and carbon dioxide absorbent caused by flooding the scrubber, and which may reach the diver's mouth through the breathing loop. The alkalinity depends on the absorbent used.

Cave - A naturally occurring cavity in bedrock, or an underwater passage not illuminated by natural daylight, large enough to be entered by a human. Statute 810.13 of the Florida legislature defines a cave as: any void, cavity, recess, or system of interconnecting passages which naturally occurs beneath the surface of the earth or within a cliff or ledge, including natural subsurface water and drainage systems but not including any mine, tunnel, aqueduct, or other man made excavation, and which is large enough to permit a person to enter. The word "cave" includes any cavern, natural pit, or sinkhole which is an extension of an entrance to a cave

Cave arrow - Directional line markers which point the way to an exit.

Cave fill - Filling a scuba cylinder to a pressure significantly above the rated safe working pressure (charging pressure). Illegal in some jurisdictions, and increases risk of catastrophic failure.

Cave reel - A reel specifically made for cave diving, used to lay and recover large lengths of cave line which is used as a guide line to find the exit or a permanent guide line.

Cavern
1.  Two or more interconnected underground rooms or passages in bedrock, each large enough to be entered by a human.
2.  The initial room of an underwater cave system that is illuminated by natural daylight.
3.  A naturally occurring cavity in bedrock or an underwater passage, large enough to be entered by a human, which is illuminated by natural daylight, or in which it is possible from all points to see the exit by natural daylight.

Cavern dive - Visibility greater than 40 feet, Maximum penetration of 130 feet, Maximum depth of 70 feet, and always within the ambient sunlight area. No passing through restrictions.

C-card or certification card - Plastic card issued to a diver by a certification agency as evidence of completed diver training and experience required for the level of certification.

CCR - Closed Circuit Rebreather

CCUBA - Closed Circuit Underwater Breathing Apparatus. Military alternative term for CCR

Ceiling - Solid overhead or decompression restriction to a safe direct vertical ascent to the surface

Cenote - A sinkhole in Mexico. Generally with vertical overhanging walls or shafts with water that open into a cave system.

Chamber dive - Simulated dive in a hyperbaric chamber pressurized to equivalent pressure to the nominal depth of the dive.

Chamber operator - Person competent to operate a diving chamber

Charging pressure - Pressure stamped on a container for a permanent gas to indicate the maximum gauge pressure (measured or corrected to 15°C) that may be applied at the time of filling.

Charles's law - Relation of volume to temperature at constant pressure of an ideal gas.

Chicken vest - Sleeveless neoprene wetsuit vest with attached hood.

Chimney - A section of a cave that is vertical or near vertical and like a shaft.

Choker - A strap around the neck of a sidemount cylinder used to hold the bolt snap closer to the neck so that the head of the cylinder stays closer to the diver's armpit. The choker can be a small webbing strap with a sliding buckle for adjustment, so it can be tightened to bring the clip closer to the neck or slacked off while in use.

Chokes - A symptom of decompression sickness manifested by shortness of breath, caused by a large number of venous gas bubbles in the lung capillaries which interfere with gas exchange.

Christmas tree ladder - A boarding ladder which has a single central rail with rigid cantilevered rungs to each side, allowing use while wearing swim fins.

Christo-lube MCG111 - an oxygen compatible lubricant suitable for use in breathing apparatus in oxygen service.

Combat swimmer - Person trained in scuba diving or swimming underwater in a military capacity which can include combat

Command signal - A signal from a diver in a team that requires a response from another diver. There are three: Are you OK, Hold, and Surface.

Compressed neoprene - Foam neoprene that has been compressed to reduce volume. Used for dry suits which are less affected by depth induced buoyancy changes.

Constant pressure blackout - A Free diving blackout which occurs while the diver maintains a near constant shallow depth, where reduction of oxygen partial pressure due to ascent is not a factor. Usually induced by pre-dive hyperventilation. Also referred to as shallow water blackout, which is an ambiguous term

Continuous decompression- Decompression without stops. Instead of a fairly rapid ascent rate to the first stop, followed by a period at static depth during the stop, the ascent is slower, but without officially stopping. Ascent rate may vary with depth, usually slowing as the depth reduces.

Control compartment - The tissue compartment that dictates the ascent profile of a given dive because it is theoretically the highest risk compartment for DCS.

Controlled emergency swimming ascent/CESA - An emergency technique for surfacing, usually when no breathable gas is available at depth. The diver fins upward while gently exhaling to keep expanding air in the lungs from causing lung expansion injuries.

Counterlung - Flexible bag or bellows in a rebreather which compensates for the change in volume in the loop during breathing.

Cracking pressure - Pressure required to open a valve. Often applied to the difference in pressure over the diaphragm of a demand valve required to open the valve to start flow. This may differ significantly from the pressure difference required to keep the valve open once flow has been initiated, and the pressure to keep the valve open may vary with flow rate.

Crushed neoprene - Proprietary material for dry suits manufactured by DUI in a process where the foam neoprene suit material is degassed by exposure to high hydrostatic pressure to reduce the volume after assembly. Less buoyancy variation with depth as the material is less compressible after the treatment.

Cyanosis - The appearance of a blue or purple coloration of the skin or mucous membranes due to the tissues near the skin surface being low on oxygen

Dalton's law - Gas law describing the relation of component pressures of gases in a mixture to the total pressure.

Dead space or dead volume
1. The volume of a breathing apparatus which holds exhaled air, which is subsequently inhaled directly. without passing through a scrubber to remove carbon dioxide and without oxygen addition.
2. The volume of inhaled air, which does not take part in gas exchange either because it remains in the conducting airways or in alveoli that are poorly perfused.

Decompression - Reduction in ambient pressure experienced by the diver during the ascent at the end of a dive or hyperbaric exposure, and the process of allowing dissolved inert gases to be eliminated from the body tissues during this reduction in pressure.

Decompression algorithm - Specified step-by step procedures used to calculate the decompression stops needed for a given dive profile. The algorithm can be used to generate decompression schedules for a particular dive profile, decompression tables for more general use, or be implemented in dive computer software.

Decompression chamber - Hyperbaric chamber used for decompressing divers and emergency therapeutic recompression.

Decompression illness - Illness caused by decompression. Includes decompression sickness and arterial gas embolism due to lung over-expansion barotrauma.

Decompression obligation - Theoretical requirement to make decompression stops during ascent based on the dive profile and the decompression model in use.

Decompression schedule - A specific ascent rate and series of increasingly shallower decompression stops that a diver uses to allow inert gases to be eliminated from the body tissues during ascent after a specific hyperbaric exposure, to reduce the risk of decompression sickness.

Decompression sickness also DCS, divers' disease, the bends or caisson disease - A condition arising from dissolved inert gases coming out of solution during decompression as bubbles in the tissues, organs and blood vessels of the body causing symptoms ranging from rashes to death.

Decompression stop - A pause during the ascent phase of a dive that a diver spends at a constant relatively shallow depth to allow safe release of inert gases from the body tissues to avoid decompression sickness.

Decompression tables - Printed cards or booklets that allow divers to determine a decompression schedule for a particular dive profile and breathing gas.

Deep stops - Decompression stops which are deeper than the deepest stops required by decompression algorithms using dissolved phase models.

Deep water blackout
1. Freediving: An ambiguous alternative term for blackout of ascent following a deep breath-hold dive, in which loss of consciousness occurs as the surface is approached, or at the surface, caused by cerebral hypoxia arising from the rapid drop in the partial pressure of oxygen in the lungs during ascent as the ambient pressure drops and the gas in the lungs expands to surface volume.
2. Scuba and surface-supplied diving: Loss of consciousness in deep air diving at depths below 50 m with no clear primary cause, associated with nitrogen narcosis, a neurological impairment with anaesthetic effects caused by high partial pressure of nitrogen dissolved in nerve tissue, and possibly acute oxygen toxicity. The term is not in widespread use at present.

Demand valve - Mechanism for providing the user with breathing gas flow only when required

Diamond Reef System - Diamond Reef System is a diving skills program that uses a set of standardized portable obstacles to train and evaluate buoyancy skills and educate scuba divers on how to interact with coral reefs.

Diluent - Gas mixture used to dilute the oxygen in the loop of a closed circuit rebreather to a partial pressure suited to the depth.

DIN fitting also DIN valve, DIN regulator, DIN thread - Usually refers to G5/8" x 14 tpi[37] parallel thread fittings used to connect a cylinder valve to a filling connection or regulator first stage. Available in 200 bar and 300 bar versions which should only be inter-connectable in safe combinations.

Divemaster - A professional level recreational diver who leads a group of less experienced or visiting divers underwater.

Divers Alert Network - A non-profit organization for assisting divers in need and medical research on recreational scuba diving safety.

Diving heavy - Practice of carrying significantly more ballast weight than necessary to neutralize buoyancy. Common in professional diving operations where the diver needs to remain in firm contact with the bottom to work effectively, and is tethered by a lifeline or umbilical to a control point at a place of safety, which is managed by a tender.

DMT - Diving medical technician: A paramedic specializing in diving related conditions.

Dome port - A domed window of optical quality glass or plastic which covers the front of an underwater camera or video housing.

Donating the primary - Rescue technique where the donor of breathing gas provides it via the primary 2nd stage –- the one from which the donor was breathing –- as it is known to be working and providing the correct gas. The donor then switches to their backup DV, often stowed under the chin by a bungee necklace with a breakaway connection.

Donating the secondary - Rescue technique where the donor of breathing gas provides it via the secondary, or octopus, 2nd stage, and continues to breathe off the primary.

Doppler bubble detection - Ultrasonic signals reflected from bubble surfaces to identify and quantify gas bubbles present in venous blood.

Downline - A rope leading from the surface down to the underwater workplace which allows a commercial diver to travel directly to and from the job site and to control rate of descent and ascent in the same way as using a shotline. Also sometimes called a jackstay. A downline used for open ocean diving is much the same as a shotline but does not reach all the way to the bottom. An open-ocean downline is weighted at the bottom, and attached to a substantial float at the surface, which may be tethered to the boat. It may be marked at intervals by knots or loops, and may be attached to decompression trapeze system. In some cases a sea anchor may be used to limit wind drift, particularly if attached to a boat with significant windage.

Downstream - In the direction of flow. Displaced from the reference point in the direction of flow

Downstream valve - Regulator valve in which the closure is downstream of the orifice. Pressure in the line tends to assist in opening the valve. When spring-loaded a downstream valve may open automatically if the pressure difference is excessive, thus functioning as a pressure relief valve

D-ring - A ring shaped like a capital D, usually of stainless steel, stitched or buckled to a diver's harness and used as an attachment point for lifeline, cylinders or other equipment.

Drift diving - Any dive where the diver is transported significantly by drifting with currents during the dive.

Drop weight - Weight used during descent and ascent, but left on the bottom at the guideline during the deep part of the dive when it is not needed due to suit compression.

Dry bag - Bag which seals in a watertight manner. Used for keeping clothes and other equipment dry in a wet environment.

Dry suit - A watertight suit worn to keep the diver dry and to provide protection from the environment. Thermal insulation may be provided by the suit or garments worn under the suit.

DSMB - An inflatable marker buoy deployed from underwater to indicate the position of a diver and to control ascent rate. Can also be used to mark a position or signal an emergency.

EAD - Equivalent Air Depth. Depth at which partial pressure of nitrogen in a Nitrox mixture at a given depth is equal to the partial pressure of nitrogen in air. Used in decompression calculations.

EAN - Enriched Air Nitrox

Ear beer - A mixture of alcohol and acetic acid in water used as a drying agent and disinfectant to rinse the ears after diving, to prevent ear infections.

Ear clearing - Equalizing the pressure in the middle and external ear by opening the Eustachian tubes. Several techniques are used.

ECCR - Electronic closed circuit rebreather. Sometime ECCCR for electronically controlled closed circuit rebreather.

Eddy current test - Method of non-destructive testing using electromagnetic induction to detect flaws in conductive materials. It is used to detect cracks in parallel neck threads of aluminum cylinders. Also called Visual Plus inspection. Required for cylinders of AA6351 alloy.

Emergency position indicating radio beacon - Emergency position-indicating radio beacons are tracking transmitters which aid in the detection and location of boats and people in distress at sea.

Emergency swimming ascent - Emergency procedure where the diver makes an ascent from depth after a breathing gas supply failure.

END - Equivalent Nitrogen Depth or Equivalent Narcotic Depth. The depth at which the narcotic effects of nitrogen in a given Trimix mixture at a given depth are equivalent to the effects of air. Used to choose nitrogen content of a Trimix breathing gas for a planned dive profile.

Equivalent air depth - A way of approximating the decompression requirements of nitrox mixtures at depth by comparison with the depth at which air would require the same decopression.

Equivalent narcotic depth - A way of expressing the narcotic effect of a breathing gas mixture at depth by comparison with the depth at which air would have a similar effect.

Exceptional exposure - A dive in which the risk of decompresssion sickness, oxygen toxixity, and/or exposure to the elements is substantially greater than on a normal working dive.

Excursion dive - Saturation diving where the divers work at a depth deeper or shallower than the saturation depth, after which they are returned to the original saturation pressure.

Face plate - Viewport of a full face mask or helmet

Farmer john - wet suit that covers the torso and legs only; it resembles a bib overalls.

Fatigue cracking - Cracking in a material resulting from multiple stress cycles below the ultimate or yield strength. Usually refers to large number of cycles.

Feather breathing - Technique for emergency breathing from a free-flowing demand valve where the diver manually controls air flow by opening and closing the cylinder valve.

FFW - Feet fresh water. Unit of pressure equal to 1/34 atm. Not a linear measure of depth.

Filtration - Process for removing impurities from a fluid. Particulates are commonly removed by passing the fluid through porous material with pore size small enough to trap the particles (e.g. micron filters). Liquids and gases are commonly absorbed or adsorbed by the surface of the filter medium (Activated carbon, Molecular sieve, Silica gel), or may be chemically combined with the medium (Sodalime) or catalytically converted (Hopcalite) into a less objectionable substance.

Fin keepers - Elastic rubber straps used to help prevent fins from falling off the diver's feet. Also known as fin retainers, fin holders, fin fasteners, fin grips, fin keeps, fin guards, flipper fixers, Y-straps, ankle straps, accessory safety straps, fix fins, grip fins and, in recognition of their French origin, as fixe-palme.

First stage - Diving regulator component which reduces gas pressure from stoage pressure in the cylinder to interstage pressure for supply to the second stage and for suit and BC inflation.

Flutter kick - fining style where the fins are alternately moved up and down by movements of the full, usually fairly straight, leg. Thrust is developed on both up and down strokes.

Fogging - Condensation of water vapor on the inside surface of a mask or helmet faceplate, reducing visibility.

Forward roll entry - Water entry technique used by scuba divers from a boat or platform too high or unsuitable for backward roll entry. The diver bends forward at the hips and waist and falls forward into the water, making a partial somersault and breaking the water with the cylinder, back and shoulders. Not suitable for heights more than about 2 meters

Free-diving - Underwater diving that does not involve the use of external breathing apparatus, but relies on a diver's ability to hold his or her breath until resurfacing. also Breath-hold diving, and apnea.

Free-flow
1. Constant flow rate air supply
2. Malfunction of a demand regulator where the valve sticks in the open position, allowing a constant rate of flow.

Free-flow helmet - A helmet where the breathing air supply is supplied at an approximately constant rate regardless of the diver's instantaneous breathing rate.

Free-flow valve - Valve on the side of a demand helmet or full-face mask which opens a free flow of breathing gas into the helmet interior, usually directed over the interior of the viewport, hence alternative term defogging valve, as it is often used to blow condensation from the inside of the viewport.

Frenzel maneuver - Technique for equalizing the middle ear by pinching the nose closed and moving the back of the tongue upwards.

Frog kick - Finning technique where thrust is developed by sweeping the fins horizontally toward each other with the fins twisted into a nearly vertical plane, with the soles facing each other, followed by a recovery stroke which develops negligible thrust where the fin blades are feathered. The legs are fairly straight during the power stroke. See also modified frog kick

Frogman - A scuba diver, particularly a military diver on an undercover mission.fsw/feet sea water - Unit of pressure equal to 1/33 atm. Not a linear measure of depth. Generally defined as the pressure exerted by a foot depth of seawater having a specific gravity of 1.027 and is approximately equal to 0.445 pounds/square inch.

Full-face mask/ffm - Diving mask covering the eyes, nose and mouth, and provides the diver with breathing gas

Gap - The space between two cave guidelines. Usually between a main guideline and a branch line.

Gap spool - A relatively short length of cave line on a spool used to bridge a gap between lines when making an excursion from the main guideline to a branch guideline.

Gaiter - legging wrapped around the calf and ankle area over a dry suit to restrict the amount of air that can get into the lower leg area. Also can reduce drag of the suit in this area by smoothing over creases and folds.

Gas blender
1. Person who mixes breathing gases for diving, filling diving cylinders with gas mixes such as nitrox or trimix.
2. Qualification to mix breathing gases for diving.

Gas blending - Mixing breathing gases for diving, filling diving cylinders with gas mixes such as nitrox or trimix.

Gas embolism - Blockage of blood vessel by a bubble of gas.

Gas panel - The control equipment for providing breathing gas to surface supplied divers via umbilicals. Primary and reserve gas is supplied to the panel through shutoff valves from a low pressure compressor or high pressure storage cylinders.

Gas switching - The procedure of changing from one breathing gas mixture to another during a dive. This may be done to avoid oxygen toxicity, hypoxia, or nitrogen narcosis, to accelerate decompression, or to avoid running out of breathing gas. Generally applied to open circuit breathing equipment, where a physical change-over of gas source is made. In closed circuit systems the gas composition is continuously controlled to follow the chosen set-point.

Gas reversal point - The depth during an ascent or decompression when the intake of dissolved gas is exceeded by outgassing.

Gauge mode- Operating mode for a personal dive computer where the decompression calculation is disabled, and the unit operated only as a timer and depth gauge. Typically used when diving with gas mixtures not supported by the algorithm, in which case decompression tables are used to monitor and control the decompression schedule.

Gauge pressure - Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure.

Gay-Lussac's law - Relation between temperature and pressure in an ideal gas for a constant volume.General gas law
1. Relation between the variables pressure, volume and temperature for a given mass of a given mixture of an ideal gas.
2. Thermodynamic equation of state for gases for the variables volume, pressure, temperature and number and atomic weight of molecules.

Glossopharangeal insufflation also buccal pumping, lung packing - A method used by freedivers for filling the lungs with more air than maximal inspiration to normal total lung capacity (TLC). After a full inhalation, the diver fills the mouth with air, while the glottis remains closed, then opens the glottis and forces this air into the lung using the cheeks and tongue to reduce the mouth volume. This may be repeated several times.

Glowstick - A single-use, translucent plastic tube containing isolated substances that, when combined, make light through chemiluminescence

Gnarly - A general purpose adjective to denote particularly difficult section of cave, which may be low, tight, silty, etc. or a combination.

Go into decompression - Incur a decompression obligation. Generally refers to having a theoretical tissue inert gas concentration that requires the diver to make staged decompression stops during ascent to avoid an unacceptable risk of symptomatic decompression sickness according to the decompression model, algorithm, tables or dive computer in use.

Golden rule - The convention in cave diving that anyone can turn the dive at any time for any reason.

Gold line - The permanent main guideline in a cave system, that usually starts well inside the cave. Often yellow or gold in colour.

Goodman handle - A handle used to carry the primary dive light head comprising a rigid slot through which the fingers and palm of the hand are extended, so that the light rests on the back of the hand, facing the direction of the extended fingers.

GPS - A satellite navigation system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites

Haldanian - Decompression models based on the principles described by John Scott Haldane.

Halocline - A strong variation in salinity over a small depth range within a body of water. Often visible as a blurred or shimmering region due to uneven refractive index.

Hand-off cylinder - A diving cylinder, complete with regulator, which can be handed off to another diver in an emergency, so that the two divers are not obliged to remain in close proximity during the exit and ascent. Transfer of a hand-off cylinder should not compromise either diver's buoyancy to the extent that they cannot make a normal, controlled ascent at neutral buoyancy.

Hang - To remain stationary at a specific depth and location, particularly when decompressing.

Hard hat diving - Surface supplied diving, generally in professional diving, either wearing a modern diving helmet or the old-style standard diving dress and brass helmet.

Harness - straps and webbing with associated buckles, D-rings and other accessories used to support the breathing apparatus and secure it to the diver. The harness often has other functions such as supporting weighting and buoyancy control systems and for recovery of the diver from the water.

Hat - Commercial diver term for diving helmet.

Hazmat diving - Diving in a known hazardous materials environment. The environment may be contaminated by hazardous materials, the diving medium may be inherently a hazardous material, or the environment in which the diving medium is situated may include hazardous materials with a significant risk of exposure to these materials to members of the diving team. Special precautions, equipment and procedures are associated with hazmat diving.

Heliox - Mixtures of helium and oxygen for use as a breathing gas.

Heliair - Trimix blends made by topping up helium with air.

Helicopter turn - Maneuver in which a (usually horizontally trimmed) diver uses small fin movements to rotate on the spot on a vertical axis.

Helium- An inert gas which is used as a component of breathing gas mixtures for deep diving.

Helium analyzer - An instrument used to identify the presence and concentration of helium in a mixture of gases

Henry's law - Description of the relation between solubility of a given gas in a given liquid as pressure varies.

HID light - High intensity discharge light: Used in cave diving light heads.

HP - High pressure, generally gas pressures in excess of 30 bar. In diving context gas working pressures do not frequently exceed 300bar, but pressures in hydraulic systems may be considerably higher.

Hogarthian configuration - A scuba combination of backplate, wing, one-piece harness with crotch-strap, regulator arrangement including long-hose primary with a necklaced secondary demand valve, and, if used with twin cylinders, an isolation manifold. Named after Willian Hogarth Main, a developer and proponent of the system.

Hog looped- A scuba configuration where the primary demand valve has a long hose which is routed under the right arm, usually tucked under a light battery canister on the waist belt of the harness, and around behind the neck to reach the mouth from the right hand side. Part of the Hogarthian configuration.

Homebrew - Amateur blended mixed gas.

Hookah also hooka - Also known as airline diving; surface supplied diving where the breathing air is supplied to the diver by a simple hose. The diver usually breathes through a mouth held demand valve.

Hook breathing - A technique used by freedivers on surfacing to reduce the risk of surface blackout. A partial exhalation is made, followed by a quick inhalation, then the diver closes the airway and pressurizes for a few seconds as if about to cough. This is repeated a few times over the first 30 seconds or so on the surface. The aim is to keep thoracic pressure slightly raised to artificially raise arterial oxygen partial pressure or prevent it from dropping in the critical seconds until newly oxygenated blood can reach the brain, and thereby prevent surface blackout. This is the same technique used by pilots during high-g maneuvers, and by mountaineers at high altitude.

Hopcalite - Catalyst sometimes used in air filters to oxidize carbon monoxide to carbon dioxide.

Hopcalite is a mixture of manganese oxide, copper oxide and a small amount of silver oxide.

Hot mix - A breathing gas mixture with a high percentage of oxygen for the depth at which it is used. Using a gas mixture with a high partial pressure of oxygen.

Hot water suit - An insulated pipe in the umbilical line, which links the diver to the surface support, carries the hot water from a heater on the surface down to the suit. The diver controls the flow rate of the water allowing him to vary the warmth of the suit.

HPNS - High-pressure nervous syndrome: A neurological and physiological diving disorder that results when a diver descends below about 500 feet (150 m) while breathing a helium–oxygen mixture.

H-valve - Cylinder valve body with two outlets and two valve mechanisms which can be independently controlled so that two regulator first stages can be fitted. Similar to Y-valve but in configuration where the second valve is parallel to the primary, though the secondary valve can sometimes be swiveled.

Hydro - Slang/abbreviation for hydrostatic testHydrostatic pressure - Pressure due to the weight of the water column above a point at depth.

Hydrostatic test - Non-destructive test to revalidate pressure vessels which uses water as a test medium. The vessel is pressurized to the test pressure and measured for permanent set.

Hydreliox - Deep diving breathing gas mixture of hydrogen, helium and oxygen.

Hydrox - Deep diving breathing gas mixture of hydrogen and oxygen.

Hypercapnia - Excessive level of carbon dioxide in the body.Hyperventilation
1.  A deliberate deep breathing to reduce blood carbon dioxide level to extend the duration of a free dive.
2.  Rapid breathing as the body's response to hypercapnia.
3.  Rapid, often shallow breathing, associated with panic.

Hypocapnia - Abnormally low tissue and blood carbon dioxide concentration.Hypothermia - A lowering of deep body temperature due to heat loss.

Hypoxia - Abnormally low tissue oxygen concentration: Insufficient oxygen in the body to support normal activities or consciousness.

Hypoxic training - prolonged underwater distance swimming or extended breath-hold intervals.

Inert gas - A gas which is not metabolically active, used to dilute the breathing gas.

Ingassing - Inert gas uptake in body tissues during a dive or other hyperbaric exposure.

Inherent unsaturation - Metabolic reduction of total gas pressure in the tissues.

Inshore diver - Colloquial term for a diver who works on inland dive sites or coastal waters not associated with the oil and gas industry. Also referred to as "civils" as much of this work is connected with civil engineering works.[4]

Integrated weight system - Any system for carrying dive weights on the buoyancy compensator or diving safety harness, avoiding the use of a separate weight harness or weightbelt.

Internal condition of cylinder - The state of the internal surface of a cylinder regarding corrosion, contamination and cracking.

Interstitial emphysema - Gas trapped in the spaces between organs after lung barotrauma.

IP - Intermediate Pressure, or Interstage Pressure. The reduced pressure between the first and second stages of a diving regulator. Also referred to as LP (Low Pressure) in this context.

Inverter line tripping line - A line attached to the top of an open parachute lifting bag and at the other end to an anchor point. Its purpose is to invert and thus empty the bag if it becomes detached from the load.

Isolation manifold - Connection between two scuba cylinders which when open allows free flow of gas in both directions between the cylinders, but has an isolation valve to block this flow.

Isolation valve - valve in an isolation manifold (q.v.) used to close the gas passage through the manifold and isolate the contents of the two cylinders. Used to prevent a leak on one cylinder from causing the other cylinder to also lose gas.

Jackstay - A line secured at both ends to serve as a support or guide.

J-cylinder - Bulk gas storage cylinder with internal volume 50 litres.

JIM suit - An atmospheric diving suit manufactured by Underwater Marine Equipment Limited.

Jocking strap - webbing strap system used with diving helmets to hold the helmet assembly down on the diver to prevent buoyancy lifting it when underwater.

Jonline - A short line used to connect to a shotline or anchor line, allowing the diver to move a short horizontal distance away to decompress. The line helps compensate for vertical movement in the anchor line or shot line due to waves.

Jump camera - A camera mounted on a frame, which when lowered to the bottom of a body of water, takes a photograph, usually of the bottom under the camera. The frame constrains the camera to a fixed camera to subject distance, resulting in photographs of uniformly sized areas of bottom, equivalent to quadrants.

Jump jacket - The Jump Jacket is a harness with integral buoyancy jacket specifically designed for commercial diving work with helmets and bells.

Jump line - A short cave line used to connect between two permanent lines that are not in contact. May also be used to search for the other end of a break in a cave line and repair the break

Jump reel jump spool - A reel or spool with a relatively short line intended to be used as a jump line (q.v.).

J-valve - Scuba cylinder valve with lever operated reserve mechanism.

kayak diving - Diving from a special purpose kayak used to get to the site where the distance from a suitable entry and exit point is inconvenient for shore diving.

K-cylinder - Bulk high pressure gas storage cylinder size designation (approximately 50 litres internal volume) Kelly Kelly tube

K-valve - Scuba cylinder valve without reserve mechanism.

Lanyard - A piece of cordage used to secure or lower things; usually it is used where there is a risk of losing the object

LARS - launch and recovery system - Mechanized system for launching and recovery of a diving bell, diving stage, submersible or ROV from a vessel, offshore platform, dockside or other platform.

Laryngospasm - An uncontrolled or involuntary muscular contraction (spasm) of the laryngeal cords which causes a partial blocking of breathing in, while breathing out remains easier.

Latent hypoxia - While freediving, an arterial pO2 which is sufficient to sustain consciousness at depth, but when ascending drops to hyoxic levels due to the reduction of ambient pressure, associated with ascent blackout.

Lay barge - Barge on which pipeline sections are assembled, welded and laid on the seabed as the barge is moved forward.

LDS - Local dive shop

LED - Light emitting diode, commonly used in dive lights.

Lifeline - A line connected securely to the diver at one end and anchored at the other end at the diving control point, which is handled by a line tender, and is used to communicate with the diver and provide a means of finding the diver for a surface standby diver, and for assisting the diver to the surface and back to the control point if necessary.

Life support life support system - Equipment vital to the short term survival of the diver. Most notably the breathing gas supply, and for saturation diving, equipment for providing a correctly pressurized environment. In some cases thermoregulatory equipment is also considered life support, and in saturation diving, all of the peripheral systems essential to maintaining a habitable saturation system.

Lift bag - A robust and air-tight bag with straps, which is used to lift heavy objects underwater by means of the bag's buoyancy.

Light head - The part of a canister light (q.v.) which emits light, and is held in the hand or mounted on the helmet.

Line marker - Line arrows, cookies and sometimes clothes pegs. Used to indicate direction to exit, midway point between exits, jumps and personal markers to identify divers on a guide line

Lipid pneumonia - A lung inflammation (pneumonia) that develops when lipids enter the bronchial tree

Live-aboard - A large boat which provides transport, accommodation and services for vacationing divers

Air-Lock - Compartment of a hyperbaric habitat or chamber which can be entered through two or more openings which can be closed and sealed, and the pressure adjusted relative to the adjacent chambers. Used to transfer personnel or equipment between areas of different pressure.

Lock-out - Exit from a pressurized environment through an air-lock

Log book - Record of dives kept as proof of experience. Optional for recreational divers, but legally required for professional divers in many jurisdictions.

Long hose - 5 ft to 7 ft inter-stage hose used on one of the regulators used by cave and other technical divers, which allows gas sharing through narrow spaces.

Longshore current - Mass transport of water along a shoreline, generally due to wave action at an angle to the shoreline.

Breathing loop - The circuit in a rebreather through which the breathing gas passes during a breathing cycle.

Loop volume - volume of the breathing loop of a rebreather.

Lost buddy drill - standardized procedure followed when a diver realizes that their buddy is not where they should be. Procedures may vary depending on the circumstances and training organizations.

Lost line drill - Standardized procedure to be followed when the guideline to the surface is lost in a penetration dive, often in conditions of low visibility and darkness.

Low impact diving - Diving with low environmental impact. Diving in a way that avoids contact with or disturbance of sensitive organisms and adversely affecting the environment.

LP - low pressure

LP compressor - Low-pressure compressor. Used for breathing air supply for surface supplied air diving.

LP cylinder - Low-pressure cylinder (US) with working pressure less than 2500 psi.

LP port - Opening on the first stage of a regulator through which regulated gas is supplied.

LST - Life support technician: A person who operates and maintains the life support systems of a saturation diving system.

Lung packing - A method used by free divers for filling the lungs with more air than maximal inspiration to normal total lung capacity (TLC). After a full inhalation, the diver fills the mouth with air, while the glottis remains closed, then opens the glottis and forces this air into the lung using the cheeks and tongue to reduce the mouth volume. This may be repeated several times.

Mammalian Diving Reflex - A reflex response to breath hold and chilling of the face diving response expressed by the cardiovascular system, which exhibits hypertension, bradycardia, oxygen conservation, arrhythmia, and contraction of the spleen

Manufacturing Code - Set of design and manufacturing rules intended to produce uniform and safe products by several manufacturers in an industry.

Marsh Marine Connector - One of the popular underwater plug connector systems for diver communications cables.

Martini's Law - Rough rule of thumb for estimating nitrogen narcosis effects based on equivalence to consumption of dry martinis: Variously quoted as one martini per 10m or one martini per 50ft depth.

Master Link (Rigging) - The large heavy duty link to which the legs of a chain sling are attached, and which is the attachment point on the sling for the lifting hook or shackle

Maze Cave - Cave structure characterized by multiple branches and changes in direction.

MCCR Manual Closed Circuit Rebreather: A closed circuit rebreather which relies on the diver to control the gas mixture in the loop.

Micro bubbles Microscopic bubbles which are not detectable by ultrasound or Doppler ultrasound, yet can affect the likelihood of DCS by slowing off-gassing.

Mid-Water - Significantly distant from the both the bottom and the surface.

Medical Lock - Small lock on a decompression chamber used for transfer of medical equipment and other supplies into and out of the chamber while the chamber remains under pressure.

Mix - Short for trimix breathing gas mixed gas Breathing gas for diving other than air, but usually implies a helium based mixture.

Mobile Diving System - portable diving system A diving system (q.v.) which is installed on a vessel or installation on a temporary basis and that is not fixed, i.e. can be demobilized, transported and re-sited. Includes surface supplied air, nitrox, HeO2 and saturation diving systems.

MOD - Maximum operating depth: Limiting depth for safety based on partial pressure of oxygen of a breathing gas mixture.

Modified Flutter Kick - Version of the flutter kick fining style which reduces risk of silting by directing thrust more directly backwards. Two techniques exist: One version has the legs bent at the knees so that the fins are placed relatively high and on average are aligned more horizontally. The other version has one fin stationary below the moving fin to deflect

Down Wash - Leg movement is restrained, and ankle movement used for precision maneuvering.

Modified Frog Kick - Version of the frog kick fining style which reduces risk of silting by directing thrust more directly backwards. Performed with bent knees and fins raised above the line of the torso. M

Monofin - A type of swim fin typically used in fin swimming and free-diving. It consists of a single surface attached to foot pockets for both of the diver's feet.

Molecular Sieve - Material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids. Molecules small enough to pass through the pores are absorbed while larger molecules are not. It is different from a common filter in that it operates on a molecular level.

Moonpool - An opening in the floor or base of the hull, platform, or chamber giving access to the water below.

MSW - meters sea water: Unit of pressure equal to 1/10 bar. Not a linear measure of depth.

Multilevel dive - A dive profile in which the diver remains in more than one distinct depth ranges for a significant period before beginning final ascent to the surface (excluding decompression stops).

Mung - A brown organic deposit usually found on the ceilings of caves which is easily dislodged by diver's exhaust bubbles and then drifts down through the water.

Multiple Stage Compressor - Compressor in which gases are compressed more than once, often with cooling between stages. Used to improve efficiency and reduce temperatures.

Mushroom Valve - A rubber non-return valve flap which is circular or oval, with a stem in the middle to attach it to the holder in the center of the grating over the orifice.

M-value - At a given ambient pressure, the M-value is the maximum theoretical value of absolute inert gas pressure that a tissue compartment can take without presenting symptoms of decompression sickness.

NACD - National Association for Cave Diving

No Decompression Limit - The maximum time which a diver can remain at a specified depth without incurring a stage decompression obligation in terms of the specified decompression tables or algorithm.

Neck Dam - The lower part of a lightweight diving helmet which includes a neoprene or latex neck seal to prevent ingress of water. The alternative is to clamp the helmet to the diver's drysuit.

Necklace - Bungee loop attached to the secondary regulator second stage, worn around the neck to store the secondary close under the chin, where it is protected and the diver is immediately aware of a free-flow. With a little adjustment and practice it is possible to pick it up by head and mouth movement alone, not needing use of a hand. The bungee is attached to the second stage by a breakaway connection, often a close-fitting loop over the mouthpiece, so that it can be moved away from the diver's head in an emergency without disturbing the primary second stage or the mask.

Neck Thread - Internal screw thread in the bore of the cylinder neck to fit a cylinder valve, which controls flow of gas into and out of the cylinder.

Neutral Buoyancy - Having a fully immersed buoyancy exactly equal to weight, so that the forces are balanced and the person or object statically remains at a constant depth. Effectively average density is equal to that of the surrounding fluid medium. The state of neutral buoyancy is typically meta stable for a compressible system.

Newtsuit - An atmospheric diving suit designed and originally built by Phil Nuytten.

Net Cutter - A handle with a hooked blade used to cut netting or cordage to free the diver from entanglement

NFCI Non-freezing cold injuries - Permanent tissue damage due to low temperature exposure without any freezing damage.

Niggles - Minor symptoms characteristic of mild decompression sickness.

Night Diving - Night diving Diving during the hours of darkness.

NiMH Nickel-metal hydride - A technology for rechargeable battery cells.

Nitrile Rubber - A synthetic elastomer used for most standard O-ring seals.

Nitrogen - The major component gas of air and many breathing gas mixtures used in diving. Important in diving as an active agent in nitrogen narcosis and decompression sickness.

Nitrogen Narcosis - Also known as narcs, inert gas narcosis, raptures of the deep, Martini effect: A reversible alteration in consciousness that occurs while breathing gases containing nitrogen under elevated partial pressure similar to alcohol intoxication or nitrous oxide inhalation, and can occur during shallow dives, but usually does not become noticeable until greater depths, beyond 30 meters.

Enriched Air Nitrox (EAN) - Nitrox Mixture of nitrogen and oxygen for use as breathing gas. Usually with oxygen percentage higher than air.

Nitrox Stick - A mixing tube used to blend oxygen with air before compressing to make nitrox breathing gas.

NOAA National Oceanographic and Atmospheric Administration

No-Mount Diving - Side mount diving § Terminology A specialized overhead-environment strategy for dealing with particularly tight restrictions which may involve divers wearing a very basic harness or simply hand-carrying cylinders.

Normoxic - 1. A breathing gas mixture with oxygen content approximating atmospheric air. 2. A breathing gas for diving which contains sufficient oxygen to minimize risk of hypoxia at atmospheric pressure.

NSS National Speleological Society