Scuba Diving Terminology: A Brief Glossary

So you have finally booked your first Scuba Dive but don’t know what it is all about and what are the different jargons associated with it. Or maybe you are one of those who did the Scuba Diving but always wondered what terms your instructors were using during the briefing. Don’t worry…I belong to the second category. Even I was unaware of many a terms that the instructors were using which I googled later. So here’s basically a brief guide on the Scuba Diving Terminology for you which gives an idea about basic terms used in underwater diving. Read on…


Actual Bottom Time (ABT)

Represents the amount of time a diver spent underwater. This time begins upon descent and ends upon the beginning of the ascent.

Adjusted No-decompression Limit

The maximum bottom time for a repetitive dive.

Air Compressor

A machine that compresses or pressurizes air to fill your cylinders before going diving.

Air Pressure

The force per unit area exerted by the weight of air.

Alternative Air Source

A secondary supply of air or other breathing gas for use by the diver in an emergency.


An absence of oxygen.

Aqua Lung

It was the first open-circuit, self-contained underwater breathing apparatus (or “SCUBA”) to reach worldwide popularity and commercial success.This class of equipment is now commonly referred to as a diving regulator or demand valve.

Archimedes’ Principle

It states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the center of mass of the displaced fluid.


Is an inert gas which is used in technical scuba diving to inflate a dry suit because it is inert and has low thermal conductivity.

Ascent Rate

The proper speed for ascending, which is no faster than 18 meters/ 60 feet per minute.

Atmosphere Absolute

The ambient pressure including the air column over the water. The air column = 1 atm at sea level.

Back Roll Entry

Leaving the dive boat by sitting on its rail/pontoon and rolling backwards into the water.


Short for barometric pressure.


Injury caused by pressure difference. It is a physical damage to body tissues caused by a difference in pressure between a gas space inside, or in contact with the body, and the surrounding gas or fluid.


Garment which provides full-length, abrasion protection when diving in conditions where a full wetsuit may not be required.


Neoprene boots worn with open heel fins.


Slang used for a scuba cylinder in United Kingdom.

Bottom Time

Time from the start of a descent until the start of a continuous ascent to the surface.

Bounce Dive

In recreational diving, 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.

Boyle’s Law

Is an experimental gas law that describes how the pressure of a gas tends to increase as the volume of the container decreases.


The diving partner

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 System

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


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

Buoyancy Compensator Device or BCD or BC

An airtight bladder worn by a diver which can be filled with air and vented to adjust and control the buoyancy of the diver.

Carbon Dioxide Poisoning

The toxic effects of carbon dioxide, occasionally caused by contaminated gas supply.

Carbon Monoxide Poisoning

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

C-Card or Certification Card

A plastic card issued to a diver by a certification agency as evidence of completed diver training and experience required for the level of certification.


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

Charles’ Law

Relation of volume to temperature at constant pressure of an ideal gas.


Machine which pressurizes gas. Generally intake gas is at ambient pressure, outlet gas at higher pressure. High pressure breathing air compressor output pressure is usually 200 to 330 bar.

Contra-indications to Diving

Conditions (usually medical) that indicate that a person should not dive.

Controlled Buoyant Lift

A rescue technique used by scuba divers to raise an incapacitated diver to the surface from depth.


Refillable compressed gas container of water capacity between 0,5 L and 150 L.

Cylinder Boot

Rubber or plastic cover for the base of a scuba cylinder to protect it from abrasion, and in the case of domed end cylinders, to allow it to stand upright.

Dalton’s Law

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

Deadman Anchor

A heavy weight used to control the rise of a lift bag after breakout, or to capsize it to prevent a runaway lift.


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 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.

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.

Defogging Solution

“Fake spit” that prevents fog from building up inside the mask during diving.


A condition where the water content of the body is reduced.

Depth Gauge

A pressure gauge calibrated to measure depth as a function of ambient pressure.

Distance Line

A line used by scuba divers as a means of returning to a safe starting point in conditions of low visibility, water currents or where pilotage is difficult.

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.

Dive Flag

A flag used to indicate that there are divers in the water. There are two versions: the international code letter flag ‘Alpha’ and the red flag with white diagonal bar.

Dive Marshall

Person who organizes and plans a group dive outing for recreational divers, assesses risk, logs divers into and out of the water and is available at the site to manage incident response.


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

Dive Profile

The variation of depth with elapsed time during a dive, often depicted as a graph.

Dive Time

The total elapsed time spent underwater during a dive.

Divers Alert Network (DAN)

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

Diving Depth

Generally the maximum depth to which the diver is exposed during a dive.

Diving Signals

Hand sign and light sign system used by scuba divers to communicate when underwater.

Diving Stage

A platform on which a diver stands which is hoisted into the water, lowered to the workplace at the bottom, and then hoisted up again to return the diver to the surface and lift him out of the water. The diving stage is particularly effective for controlling rate of descent and ascent.

Diving Medical Technician (DMT)

A paramedic specializing in diving related conditions.

Donkey Dick

Slang term for the corrugated buoyancy compensator inflation and deflation hose.

Down Line

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.

Downstream Valve

Valve in which the closure is downstream of the orifice.


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 Dive

Scuba diving in a current.

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 Filling

Filling scuba cylinders without the use of a water bath for cooling.

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.

Equivalent Air Depth (EAD)

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.

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 or Ear Equalization

Equalizing the pressure in the middle and external ear by opening the Eustachian tubes. Several techniques are used for the same.

Exceptional Exposure

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

Eustachian Tube

Hollow structure of bone and cartilage extending from the middle ear to the rear of the throat. By permitting air to leave or enter the middle ear, the tube equalizes air pressure on either side of the eardrum

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.


Worn on the feet to increase maneuverability and allow for faster swimming.

First Stage

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

Flutter Kick

Finning 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.


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

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.

Frenzel Maneuver

Technique for equalising the middle ear by pinching the nose closed and moving the back of the tongue upwards.


A scuba diver, particularly a military diver on an undercover mission.

Full Face Mask

Diving mask covering the eyes, nose and mouth, and provides the diver with breathing gas.

Gas Blender

Person who mixes 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.


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.


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.


A tank valve with 2 outlets.

Hyperbaric Evacuation System

System for evacuating divers under pressure from a saturation system in an emergency.

Hyperbaric Lifeboat

A lifeboat with a hyperbaric chamber and life support system built into it for evacuating saturation divers in an emergency.


Additional filter to produce air of oxygen compatible quality. This may be built into the filling system or temporarily connected when required.


A deliberate deep breathing to reduce blood carbon dioxide level to extend the duration of a free dive.

Inert Gas

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


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

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.


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

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.


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.


Scuba cylinder valve with lever operated reserve mechanism.


Bulk high pressure gas storage cylinder size designation (approximately 50 litres internal volume)


Scuba cylinder valve without reserve mechanism.


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 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 thermo-regulatory equipment is also considered life support, and in saturation diving, all of the peripheral systems essential to maintaining a habitable saturation system.

Lifting Bag

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

Line Trap

Narrow gap where the guide line passes but divers can not get through.

Loop Volume

Volume of the breathing loop of a rebreather.

LP Compressor

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

Marsh Marine Connector

One of the popular underwater plug connector systems for diver communications cables.

Maximum Operating Depth (MOD)

Limiting depth for safety based on partial pressure of oxygen of a breathing gas mixture.


A type of swimfin 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.

Negative Buoyancy

Buoyancy less than weight. Insufficient upward force due to buoyancy to keep afloat or remain at constant depth


Synthetic elastomer used in the form of foamed sheets as the material for most wetsuits and some drysuits.


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

Octopus Breathing

Sharing air using an octopus regulator.

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.

Open Circuit

Breathing apparatus which discharges exhaled gas into the environment, without any further use.


Entering a region with no direct vertical access to the surface, such as a cave or the interior of a wreck.

Personal Locator Beacon

Radio beacons for personal use which are intended to indicate a person in distress who is away from normal emergency services.

Pony Cylinder

Relatively small scuba set carried as an independent alternative breathing gas source by a scuba diver.


Breathing apparatus which recycles most of the exhaled gas, removing carbon dioxide and topping up oxygen before the gas is breathed again.

Reclaim System

System for recovering helium based breathing gas used by divers and recycling it.

Recreational Diving

Recreational diving or sport diving is a type of diving that uses scuba equipment for the purpose of leisure and enjoyment.


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

Rule of Thirds

Cave and wreck penetration breathing gas management convention where no more than one third of the gas in a cylinder may be used on the inward part of the dive, and the other two thirds is kept for exit: One third for the planned exit, and one third in case of an emergency.

Run Time

Time elapsed since the start of a dive.

Safe Second

Obsolete term for backup regulator.

Safety Stop

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


Diver propulsion vehicle used by scuba divers to increase range underwater.


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.

Skip Breathing

Breathing pattern where the diver holds each breath a while to conserve breathing gas.

Spring Strap

Fin strap using a stainless steel spring to secure the fin to the foot.


A full one piece wetsuit that covers the torso and the full length of the arms and legs.

Tethered Diving

Diving with a lifeline between the diver and a surface tender.

Thumb The Dive

Terminate the dive by signalling exit to surface at a time or place other than the planned turning point.

Toynbee Manoeuvre

Method of equalizing the middle ears by pinching the nose and swallowing.


Mixture of three gases for breathing. Oxygen, nitrogen and helium are the gases used.

Turn The Dive

Start the return on a dive which has reached the planned turning point in terms of depth, time, gas supply or distance.

Valsalva Maneuver

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


A type of dizziness, where there is a feeling of motion when one is stationary.


Glass or plastic window on a diving helmet or full face mask.

Wall Diving

Diving along the face of a near vertical cliff wall, particularly if the bottom is below the range of the diver’s equipment and certification. This requires good buoyancy control.

Weight Belt

Ballasted waist belt worn by divers to compensate for excess buoyancy. For scuba and freediving, usually easily removed to establish positive buoyancy in an emergency.


A close fitting, thermally-insulating, foam neoprene diving suit that allows a limited volume and movement of water inside the suit.

Wreck Diving

Recreational or technical diving on and inside of shipwrecks.


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 H-valve but in Y configuration. Also known as Slingshot valve.

Zip Tie

Self-locking plastic strip used to connect objects together.

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