Glossary 152 terms
Scuba Diving Glossary
This scuba diving glossary defines the terms and abbreviations you'll hear on boats, in courses and in dive briefings. Where a term has its own article, the definition links to it.
Terms are listed alphabetically. Abbreviations are listed under their own letters, with the full term in the definition. For full articles rather than short definitions, use the A–Z index.
A
- Absolute pressure
- Total pressure including the atmosphere above the water: 1 bar at the surface, plus about 1 bar per 10 m of seawater. See water pressure at depth.
- Advanced open water
- A common second-level recreational course that introduces deeper, navigation and specialty dives.
- Air integration
- A dive computer that also shows tank pressure, through a hose or a wireless transmitter.
- Alternate air source
- A spare second stage or separate supply a buddy can breathe from if they run out of gas. See scuba regulator.
- Altitude diving
- Diving in lakes well above sea level, which needs adjusted tables or computer settings.
- Aluminium 80
- The most common rental tank: an aluminium cylinder holding about 77 cubic feet of air at 207 bar (3,000 psi). See scuba tanks.
- Archimedes' principle
- An object in water is pushed up by a force equal to the weight of the water it displaces. See Archimedes' principle in diving.
- Arterial gas embolism
- A gas bubble in the arteries, usually from a lung overexpansion injury on ascent. A medical emergency.
- Ascent rate
- How fast a diver rises. Many computers warn above about 9 to 10 m (30 to 33 ft) per minute. See safety stop.
- ATA
- Atmospheres absolute, a unit of absolute pressure. 1 ATA is the pressure at sea level.
B
- Back-inflate BCD
- A buoyancy control device with all its air cell behind the diver.
- Backplate
- A rigid metal or plastic plate worn with a harness and wing, common in technical diving.
- Backup light
- A second light carried in case the primary light fails, essential at night and in overhead environments.
- Bar
- A unit of pressure close to one atmosphere. 1 bar is about 14.5 psi.
- Barotrauma
- Injury caused by pressure differences, most often in the ears, sinuses or lungs. See ear equalization.
- BCD
- Buoyancy control device, the inflatable vest or wing that holds the tank and controls buoyancy. See buoyancy control device.
- Bends
- Common name for decompression sickness. See decompression sickness.
- Bioluminescence
- Light made by living things, such as plankton that glow when disturbed at night.
- Bottom time
- The time from leaving the surface to starting the final ascent, used in table planning.
- Boyle's law
- At constant temperature, gas volume is inversely proportional to pressure. See Boyle's law in scuba diving.
- Buddy
- Your dive partner, responsible for diving with you and helping in a problem.
- Buddy check
- A short pre-dive check of both divers' gear. See buddy check.
- Buoyancy
- The upward force water exerts on a diver. Divers aim for neutral buoyancy for most of a dive.
- BWRAF
- Memory aid for the buddy check: BCD, Weights, Releases, Air, Final OK.
C
- Cavern
- The area of a cave within sight of daylight from the entrance. See cave diving.
- CCR
- Closed-circuit rebreather, a unit that recycles exhaled gas, removes carbon dioxide and adds oxygen.
- Cenote
- A natural sinkhole, especially in Mexico's Yucatán Peninsula, often leading to flooded caves.
- CNS clock
- A percentage a computer shows to track oxygen exposure against published limits. See oxygen toxicity.
- CNS oxygen toxicity
- Oxygen poisoning of the brain at high partial pressure, which can cause convulsions.
- Compressed air
- Filtered atmospheric air in a scuba tank: about 21 percent oxygen and 78 percent nitrogen. See compressed air.
- Conservatism
- A dive computer setting that shortens limits for an extra safety margin.
- Console
- A housing on the regulator hose holding the pressure gauge and sometimes a depth gauge, compass or computer.
- Current
- Moving water. Divers use it on drift dives. See drift diving.
D
- Decompression sickness
- Illness caused by nitrogen bubbles forming in the body after a dive. See decompression sickness.
- Decompression stop
- A required stop during ascent after exceeding a no-decompression limit.
- Demand valve
- The valve in a second stage that delivers gas only when the diver inhales.
- Depth gauge
- An instrument that shows current depth, now usually part of a dive computer.
- DIN
- A threaded regulator and valve connection, common in Europe and technical diving. See scuba tanks.
- Dive computer
- A device that tracks depth and time and calculates no-decompression limits. See dive computer.
- Dive flag
- A flag showing divers are in the water: the red flag with a white diagonal stripe, or the blue and white Alpha flag.
- Dive log
- A record of each dive's date, site, depth, time and notes.
- Dive plan
- The agreed maximum depth, time, gas limits and procedures for a dive.
- Divemaster
- The first professional level in recreational diving, qualified to guide certified divers.
- Drift dive
- A dive in which the current carries divers along a site while the boat follows. See drift diving.
- Drysuit
- A watertight suit that keeps the diver dry, with warmth from undergarments. See wetsuits and drysuits.
- DSMB
- Delayed surface marker buoy, an inflatable tube sent up on a line to show the boat where divers are.
E
- EAD
- Equivalent air depth, the depth on air with the same nitrogen loading as a nitrox mix at a given depth.
- EANx
- Enriched air nitrox; the x is the oxygen percentage, as in EAN32. See enriched air nitrox.
- Enriched air
- Nitrox with more than 21 percent oxygen.
- Equalizing
- Adding air to the ears, sinuses or mask to match the surrounding pressure. See ear equalization.
- Eustachian tube
- The passage between the middle ear and throat that divers open to equalize.
- Exposure suit
- Any suit worn for warmth and protection: rash guard, wetsuit, semi-dry or drysuit.
F
- Fin pivot
- A buoyancy drill in which a diver rises and falls on their fin tips using only breathing.
- Fins
- Blade-shaped footwear that lets divers swim efficiently.
- First stage
- The part of the regulator on the tank that reduces tank pressure to intermediate pressure. See scuba regulator.
- Free-flow
- A regulator delivering gas continuously instead of on demand.
- Freediving
- Diving on a single breath, without scuba.
- Frenzel
- An equalizing method using the tongue to push air into the Eustachian tubes.
- Frog kick
- A sweeping kick that pushes water backward, used to avoid stirring up silt. See neutral buoyancy.
G
- Gas management
- Planning how much gas you'll use, when to turn the dive and what reserve to keep.
- Gauge pressure
- Pressure counted from zero at the surface, ignoring the atmosphere.
- Guideline
- A continuous line laid from open water into a cave or wreck to lead divers out.
H
- Hand signals
- Gestures divers use to communicate underwater. See scuba hand signals.
- Heliox
- A breathing gas of helium and oxygen. See trimix and heliox.
- Henry's law
- The amount of gas that dissolves in a liquid is proportional to that gas's pressure.
- HPNS
- High-pressure nervous syndrome, tremors and nausea from fast compression on helium at extreme depth.
- Hydrostatic test
- A pressure test of a scuba tank, required every few years.
- Hyperbaric chamber
- A sealed chamber that raises pressure, used to treat decompression sickness.
- Hypothermia
- A dangerous drop in core body temperature.
- Hypoxic mix
- A gas with too little oxygen to breathe safely at the surface, used only at depth.
I
- Inflator
- The low-pressure hose and buttons used to add or release air from a BCD.
- Intermediate pressure
- The pressure in the hose between the first and second stages, typically about 9 to 10 bar above ambient.
L
- Light zone
- The area inside a wreck or cave where daylight from an opening is still visible.
- Live-aboard
- A boat divers live on for several days, diving from it.
- Low on air
- The signal of a fist against the chest, meaning it's time to end the dive.
- Lung overexpansion injury
- Damage from expanding air in the lungs when a diver holds their breath on ascent. See Boyle's law in scuba diving.
M
- Manifold
- A connection that joins two tanks so a diver can breathe from both.
- Mask
- A face mask with a glass lens and nose pocket, giving clear vision underwater.
- Mask squeeze
- Pressure on the face from a mask that hasn't been equalized by exhaling through the nose.
- MOD
- Maximum operating depth, the depth at which a gas reaches its oxygen partial pressure limit. See oxygen toxicity.
- Multilevel dive
- A dive spent at progressively shallower depths, which computers credit with longer limits.
N
- Narcosis
- See nitrogen narcosis.
- NDL
- No-decompression limit. See no-decompression limits.
- Negative buoyancy
- The state of sinking.
- Negative entry
- Entering with an empty BCD and descending straight away, common on drift dives.
- Neoprene
- A rubber foam full of gas bubbles, used for wetsuits.
- Neutral buoyancy
- Neither rising nor sinking. See neutral buoyancy.
- Night dive
- A dive made after dark with lights. See night diving.
- Nitrogen narcosis
- A drunk-like effect of breathing nitrogen at depth. See nitrogen narcosis.
- Nitrox
- A mix of nitrogen and oxygen, usually meaning enriched air. See enriched air nitrox.
- No-decompression limit
- The longest time at a depth from which a diver can ascend directly to the surface. See no-decompression limits.
- Normoxic trimix
- Trimix with enough oxygen to breathe safely at the surface.
O
- Octopus
- A spare second stage on a longer hose, used as an alternate air source.
- OK signal
- A circle made with thumb and index finger underwater, or an arm curved over the head at the surface.
- Open water
- Water with direct access to the surface, as opposed to an overhead environment. Also the name of the first scuba certification.
- Out of air
- The signal of a flat hand across the throat, an emergency calling for an alternate air source.
- Overhead environment
- Any place without a direct path to the surface, such as a cave or wreck interior.
- Overpressure valve
- A valve that stops a BCD from overinflating.
- Overweighting
- Carrying more lead than needed, which makes buoyancy harder and raises air use.
- Oxygen analyser
- A device to measure the oxygen percentage in a tank before a nitrox dive.
- Oxygen toxicity
- Harm from breathing oxygen at too high a partial pressure. See oxygen toxicity.
P
- Partial pressure
- The share of total pressure from one gas: its fraction times the absolute pressure.
- Penetration
- Entering the inside of a wreck or cave. See wreck diving.
- PFO
- Patent foramen ovale, a small opening between heart chambers linked to some forms of decompression sickness.
- PO2
- Partial pressure of oxygen, kept at or below 1.4 bar while working and 1.6 bar at most.
- Pony bottle
- A small extra tank with its own regulator, carried as an independent backup.
- Positive buoyancy
- The state of floating or rising.
- Pressure gauge
- An instrument showing the gas pressure left in the tank, also called an SPG.
- Primary light
- A diver's main light on night, cave and wreck dives.
- Psi
- Pounds per square inch, a unit of pressure. 1 bar is about 14.5 psi.
- Pulmonary oxygen toxicity
- Lung irritation from long exposure to raised oxygen.
R
- Rash guard
- A thin top or suit giving sun and scrape protection in warm water.
- Rebreather
- A breathing unit that recycles exhaled gas instead of releasing it as bubbles.
- Recompression
- Treatment of decompression sickness by raising pressure in a chamber.
- Recreational diving
- No-decompression diving within about 40 m (130 ft) on air or nitrox.
- Reef hook
- A hook on a line used to hold position in a current, placed only on dead rock.
- Regulator
- The device that reduces tank pressure and delivers gas on demand. See scuba regulator.
- Repetitive dive
- A dive made while nitrogen from an earlier dive is still in the body.
- Rescue diver
- A recreational course focused on preventing and managing diving emergencies.
- Reverse block
- Air trapped in the ear or sinus that can't escape on ascent.
- Rule of thirds
- A gas rule: one third in, one third out, one third in reserve. See cave diving.
S
- Safety stop
- A recommended pause, usually 3 minutes at 5 m (15 ft), before surfacing. See safety stop.
- Saturation diving
- Commercial diving in which divers live under pressure for days or weeks.
- Scuba
- Self-contained underwater breathing apparatus.
- Second stage
- The part of the regulator in the diver's mouth. See scuba regulator.
- Semi-dry suit
- A thick wetsuit with tight seals that lets in very little water.
- Sidemount
- A configuration with tanks worn at the diver's sides instead of on the back.
- Silt-out
- Loss of visibility caused by stirred-up sediment.
- Sinus squeeze
- Pain from a blocked sinus that can't equalize.
- Slate
- A small writing board used to communicate underwater.
- Snorkel
- A breathing tube for swimming face down at the surface.
- SPG
- Submersible pressure gauge, which shows tank pressure.
- Surface interval
- The time spent at the surface between dives.
- Surface marker buoy
- A buoy or inflatable tube that shows the boat where divers are.
T
- Tank
- A high-pressure cylinder that stores breathing gas. See scuba tanks.
- Tank banger
- A device that knocks on a tank to make a sound and get attention.
- Technical diving
- Diving beyond recreational limits, using decompression, overhead environments or mixed gases.
- Thermocline
- A layer where water temperature changes sharply with depth.
- Toynbee
- An equalizing method: pinch the nose and swallow.
- Trim
- A diver's body angle in the water, ideally horizontal.
- Trimix
- A breathing gas of oxygen, helium and nitrogen. See trimix and heliox.
- Turn pressure
- The tank pressure at which divers agree to turn the dive around.
V
- Valsalva
- An equalizing method: pinch the nose and blow gently against it.
- Visibility
- How far a diver can see horizontally underwater. See light and color underwater.
- Visual inspection
- A regular check of a tank's inside and outside for damage and corrosion.
W
- Weight belt
- A belt that holds lead weights, with a quick-release buckle.
- Weighting
- Choosing the right amount of lead to be neutral with a near-empty tank.
- Wetsuit
- A neoprene suit that insulates with a thin, warmed layer of water. See wetsuits and drysuits.
- Wing
- A back-mounted air cell used with a backplate and harness.
- Wreck dive
- A dive on a sunken ship, aircraft or other structure. See wreck diving.
Y
- Yoke
- A regulator connection that clamps over the tank valve, also called A-clamp or INT.