Glossary of diving terminology

Please see below a glossary of scuba diving terminology, here you can find the common terms that will help you better prepare you for your dive training.


Alternative air source

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

Ambient pressure

Pressure of the surroundings.

Arterial gas embolism

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

Ascent rate

The rate at which depth is reduced at the end of a dive. An important component of decompression.

Back gas

Breathing gas carried by a scuba diver in back mounted cylinders. Generally the primary breathing gas for the bottom or longest sector of a dive.

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.


Metric unit of pressure commonly used in diving, equal to 100 kiloPascal, and nearly equal to standard atmospheric pressure.


Injury caused by pressure difference.

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.

Buoyancy control

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

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.

Buddy check

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

Carbon dioxide poisoning

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

Controlled emergency swimming ascent (CESA)

Emergency technique for surfacing, usually when no breathable gas is available at depth.

Decompression sickness

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.

Deep water blackout

Loss of consciousness caused by cerebral hypoxia on ascending from a deep breath-hold dive, when the swimmer does not necessarily experience an urgent need to breathe.

DIN fitting

Usually refers to G5/8″ x 14 tpi[17] parallel thread fittings used to connect a cylinder valve to a filling connection or regulator first stage.

Dive computer

A device used by a scuba diver to measure the time and depth of a dive so that a safe ascent profile can be calculated and displayed so that the diver can avoid decompression sickness.

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.

Delayed surface marker buoy (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.

Free flow

Malfunction of a demand regulator where the valve sticks in the open position, allowing a constant rate of flow.

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.

Gas embolism

Blockage of blood vessel by a bubble of gas.

LP port

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

No decompression limit (NDL)

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.

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.

Nitrox, Enriched Air Nitrox (EAN)

Mixture of nitrogen and oxygen for use as breathing gas. Usually with oxygen percentage higher than air.

Octopus regulator

A secondary demand valve fitted to a first stage diving regulator for use as an alternative air source for another diver in case of an emergency.

Out of air (OOA)

An emergency situation where the supply of breathing gas to the diver has stopped.

Partial pressure

The pressure that a component gas of a gas mixture would exert if it alone was present in the volume occupied by the gas mixture.

Recreational diving

Recreational diving or sport diving is a type of diving that uses scuba equipment for the purpose of leisure and enjoyment. In some diving circles, the term “recreational diving” is used in contradistinction to “technical diving”, a more demanding aspect of the sport which requires greater levels of training, experience and equipment. In other circles, technical diving is considered a subset of recreational diving as opposed to professional diving, which is done as part of the diver’s work.


A mechanism for controlling the output pressure of a high pressure gas supply.

Reserve gas

Gas which is not intended to be used during the dive, and is reserved for contingencies.

Residual nitrogen

Nitrogen in excess of normal atmospheric saturation remaining in the diver’s tissues after a dive.

Residual nitrogen time

Time penalty in a repetitive dive equivalent to time at depth which would produce the residual nitrogen in the diver at the start of the dive.

Run time

Time elapsed since the start of a dive.

SAC rate

Surface Air Consumption rate: A measure of air consumption in units of pressure over time, usually psi/minute, adjusted to surface pressure, used to estimate air endurance of a cylinder of specific size. Useful for those who work in imperial units.[42] SAC has a constant value for a given diver and represents gas used on the surface at rest.

Safety stop

A voluntary (not required by the decompression schedule) additional decompression stop intended to reduce risk of decompression sickness.


Self Contained Underwater Breathing Apparatus. May be open or closed circuit.

Second stage

The part of a diving regulator which provides pressure reduction from intermediate pressure to ambient pressure on demand. Demand valve.


Gauge attached to the first stage regulator and used to monitor pressure remaining in the diving cylinder.

Surface interval

The time spent by a diver at surface pressure after a dive during which inert gas which was still present at the end of the dive is further eliminated from the tissues.

Task loading

A multiplicity of responsibilities leading to an increased risk of failure on the part of the diver to undertake some key basic function which would normally be routine for safety

Technical diving

An extension of the scope of recreational scuba diving to applications with greater technical complexity and higher inherent risk. Definitions vary, but diving with multiple breathing gases, helium based gases, closed circuit rebreathers, or under extensive overheads are generally considered as technical diving. There is no sharp distinction from other forms of recreational diving.

Tissue compartments

Imaginary tissues which are designated as fast and slow to describe the rate of saturation.

Tissue half times

The time it takes for the tissue to take up or release 50% of the difference in dissolved gas capacity at a changed partial pressure.

Valsalva maneuver

Technique for equalising the middle ear by moderately forceful attempted exhalation against a closed mouth and blocked nose

Yoke fitting/valve

A fitting or valve used to connect a regulator or filling whip to a diving cylinder using the “international” connection.