Scuba diving glossary

91 terms

Absolute pressurePhysics

The total pressure on a diver — the water pressure from depth plus the one atmosphere pushing down from the air above. Measured in atmospheres absolute (ata).

Air consumption (SAC rate)Planning

How fast you breathe through your air, often expressed as a Surface Air Consumption rate. Relaxed breathing and good buoyancy make your air last longer.

Air embolism (AGE)Physiology

A serious injury where air forced from over-expanded lungs enters the bloodstream. It's why you must never hold your breath and always ascend slowly.

Air sharingSafety & Skills

Breathing from a buddy's alternate air source in an out-of-air situation, then ascending together slowly — a core emergency skill.

Alternate air source (octopus)Equipment

A spare second-stage regulator a diver carries so a buddy can breathe from the same tank in an out-of-air emergency.

Ambient pressurePhysics

The pressure of the surrounding water at your current depth. It increases by about one atmosphere for every 10 m (33 ft) you descend.

Archimedes' principlePhysics

The rule behind buoyancy: an object is pushed up by a force equal to the weight of the water it displaces. It's why a full BCD floats you.

Ascent ratePlanning

How quickly you rise toward the surface. Divers ascend slowly — no faster than about 18 m (60 ft) per minute — to let nitrogen leave the body safely.

Atmosphere / ata / barPhysics

Units of pressure. One atmosphere (about 1 bar) is the pressure of the air at sea level; underwater you add another atmosphere every 10 m.

Backward rollEnvironment

A water entry from a small or low boat: you sit on the edge and roll backward, holding your mask and regulator in place.

BarotraumaPhysiology

A pressure injury to an air space such as the ears, sinuses or lungs, caused when the pressure inside and outside the space isn't equalized.

BCD (buoyancy control device)Equipment

The inflatable jacket a diver adds air to or vents to float, sink or hover. It's the main tool for controlling buoyancy underwater.

Boat diveEnvironment

A dive made from a boat, using entries like the giant stride or backward roll and often involving currents and surface marker buoys.

Bottom timePlanning

The total time from starting your descent to beginning your final ascent. It's a key input for staying within your no-decompression limit.

Boyle's LawPhysics

The physics rule that a gas shrinks under pressure and expands as pressure drops. It explains why you equalize on descent and never hold your breath on ascent.

Buddy check (BWRAF)Safety & Skills

The pre-dive check done with your buddy — BCD, Weights, Releases, Air, Final check — to catch problems before you descend.

Buddy systemSafety & Skills

Diving in pairs so each diver can help the other — sharing air, watching for problems and assisting in an emergency. A core safety practice.

BuoyancyPhysics

The tendency to float or sink in water. Divers aim for neutral buoyancy, where they neither rise nor sink and can hover effortlessly.

Carbon dioxide (CO₂) buildupPhysiology

A rise in CO₂ from shallow or skip breathing, causing headaches, breathlessness and panic. Slow, deep, continuous breaths prevent it.

CESA (controlled emergency swimming ascent)Safety & Skills

A last-resort skill: swimming calmly to the surface while exhaling continuously when out of air and no buddy is near.

Charles's LawPhysics

Gas expands when heated and contracts when cooled. It's why a cylinder's pressure reads a little higher after filling (warm) and lower once it cools.

CompassEquipment

A navigation instrument for holding a heading and finding your way back underwater, where landmarks and the surface aren't always visible.

ConsoleEquipment

A cluster that groups your gauges — typically the SPG, and often a depth gauge and compass or dive computer — on one hose.

CurrentEnvironment

Moving water that can push you along a dive site. Divers plan to start against it, drift with it, or avoid fighting it to save air and energy.

Dalton's LawPhysics

The total pressure of a gas mix is the sum of the pressures of each gas in it. It underpins partial pressures, oxygen limits and narcosis.

Decompression sickness (the bends)Physiology

An injury caused when nitrogen forms bubbles in the body after ascending too fast or staying too long at depth. Prevented by diving within limits and ascending slowly.

Decompression stopPlanning

A required pause during ascent to off-gas nitrogen when a dive has exceeded no-stop limits. Recreational divers plan to avoid needing these.

DehydrationPhysiology

Low body fluid, which thickens the blood and raises the risk of decompression sickness. Divers stay well hydrated before and after diving.

Density (of gas)Physics

How tightly packed the gas is. Air gets denser with depth, so each breath contains more of it — which is why you use air faster deeper.

Depth gaugeEquipment

An instrument showing your current and maximum depth, so you can stay within your planned and certified limits.

DIN and yoke valvesEquipment

The two ways a regulator attaches to a cylinder. Yoke (A-clamp) clamps over the valve; DIN screws in and is common for higher-pressure tanks.

DisplacementPhysics

The volume of water an object pushes aside. The more water you displace for your weight, the more buoyant you are.

Dive computerEquipment

A wrist or console device that tracks your depth and time and continuously calculates your remaining no-decompression limit, ascent rate and safety stop.

Dive tables (RDP)Planning

Printed tables (like a Recreational Dive Planner) used to calculate no-decompression limits and surface intervals before dive computers became common.

Down currentEnvironment

A downward-moving current near walls or drop-offs that can pull a diver deeper. You escape it by swimming away from the wall and adding buoyancy.

Drift diveEnvironment

A dive where you let the current carry you along while the boat follows your bubbles or surface marker — relaxing and efficient when planned well.

DrysuitEquipment

A sealed exposure suit that keeps you completely dry using seals at the neck and wrists, for cold water. It requires a short specialty course to use.

EqualizationPhysiology

Gently adding air to the middle ears (and other air spaces) on descent to balance the rising water pressure and prevent pain or injury.

Equivalent air depth (EAD)Planning

The depth at which air would give the same nitrogen exposure as your nitrox mix — used to plan nitrox dives with standard tables.

FinsEquipment

Foot blades that turn slow, powerful leg kicks into efficient propulsion, leaving your hands free and saving air.

First stageEquipment

The part of the regulator that attaches to the tank and reduces the very high tank pressure to an intermediate pressure for the hoses.

Gauge pressurePhysics

Pressure measured relative to the surface, ignoring the atmosphere. At 10 m the gauge pressure from water is 1 bar, but the absolute pressure is 2 ata.

Giant stride entryEnvironment

A water entry from a stable platform or dock: you take one big step out, holding your mask and regulator, and enter feet-first.

Hand signalsSafety & Skills

The standard gestures divers use to communicate underwater — including the OK circle, and the thumbs-up that means 'go up', not 'good'.

Henry's LawPhysics

The more pressure a gas is under, the more of it dissolves into a liquid — like your blood. It explains why you absorb nitrogen at depth and must ascend slowly.

HypothermiaPhysiology

Dangerous loss of body heat from cold water. Because water pulls heat away fast, divers dress for the temperature and end the dive if they get cold.

Low-pressure inflator (LPI)Equipment

The button-operated hose that feeds air from your tank into the BCD so you can add buoyancy at the press of a button.

Lung over-expansion injuryPhysiology

Damage from expanding air stretching the lungs on ascent — the reason for the golden rule: breathe continuously and never hold your breath.

MaskEquipment

The sealed lens that traps an air space in front of your eyes so you can see clearly underwater. It must be equalized on descent.

Mask clearingSafety & Skills

The skill of removing water from a flooded mask by exhaling through the nose while pressing the top of the frame — practised until it's second nature.

Mask squeezePhysiology

A pressure imbalance inside the mask on descent. Divers prevent it by exhaling a little air through the nose into the mask.

Maximum operating depth (MOD)Planning

The deepest you can safely take a given nitrox mix before oxygen toxicity becomes a risk. Richer mixes have shallower MODs.

Multilevel divePlanning

A dive planned in stages at different depths — deepest first, then shallower — to get more bottom time while managing nitrogen.

Neutral buoyancySafety & Skills

The balanced state where you neither rise nor sink but hover effortlessly. Achieved with the BCD and fine-tuned with your breathing.

Nitrogen narcosisPhysiology

A reversible, alcohol-like impairment some divers feel at depth (usually below 30 m / 100 ft) from breathing nitrogen under pressure. It clears on ascending.

Nitrox (EANx)Planning

Breathing gas with more oxygen and less nitrogen than air. It can extend no-decompression limits but has its own depth limit due to oxygen, and needs a short specialty course.

No-decompression limit (NDL)Planning

The maximum time you can stay at a given depth and still ascend directly (with a safety stop) without required decompression stops. It shrinks as depth increases.

Off-gassingPhysiology

The process of nitrogen leaving your body during and after a dive as pressure drops. A slow ascent, safety stop and surface interval all help it happen safely.

On-gassingPhysiology

The absorption of nitrogen into your tissues while at depth. The deeper and longer you go, the more you on-gas — which limits your no-stop time.

Oxygen toxicityPhysiology

A risk when the partial pressure of oxygen gets too high, mainly a concern with nitrox below its maximum operating depth. Managed by staying within depth limits.

Partial pressurePhysics

The share of total pressure contributed by one gas in a mix. As you descend, the partial pressures of oxygen and nitrogen rise, which matters for narcosis and oxygen limits.

Pre-dive safety checkSafety & Skills

A final head-to-toe check of your own and your buddy's gear before entering the water, ensuring air is on and everything works.

Pressure groupPlanning

A letter on a dive table representing how much residual nitrogen you have after a dive — it sets your limits for the next dive.

Recreational depth limitPlanning

The 40 m (130 ft) maximum for recreational diving; new Open Water divers are usually limited to 18 m (60 ft) until further training.

RefractionPhysics

The bending of light as it passes from water into your mask, which magnifies objects so they look about a third larger and closer than they really are.

RegulatorEquipment

The device that reduces high-pressure tank air to a breathable pressure and delivers it on demand each time you inhale.

Regulator recoverySafety & Skills

The skill of finding and replacing your second stage if it falls from your mouth underwater, so you can keep breathing calmly.

Repetitive divePlanning

Any dive made before your body has fully off-gassed from a previous one. It starts with residual nitrogen, so its no-stop limit is shorter.

Residual nitrogenPhysiology

Leftover nitrogen still in your body from a previous dive. It reduces your no-decompression limit on the next dive until enough surface time passes.

Residual nitrogen timePlanning

A time penalty added to a repetitive dive to account for leftover nitrogen, as if you'd already been at that depth for those minutes.

Reverse blockPhysiology

Trapped, expanding air in an ear, sinus or tooth that can't escape on ascent, causing pain. Slowing or pausing the ascent lets it equalize.

Safety stopPlanning

A precautionary pause, usually 3 minutes at 5 m (15 ft), made near the end of a dive to let extra nitrogen off-gas before surfacing.

Second stageEquipment

The mouthpiece part of the regulator that you breathe from; it delivers air at ambient pressure only when you inhale.

Shore diveEnvironment

A dive entered from land, walking or wading in from a beach or shoreline, which calls for good timing with surf and surge.

SnorkelEquipment

A breathing tube for the surface, letting you watch below and conserve tank air while swimming out or resting on top.

SPG (submersible pressure gauge)Equipment

The gauge that shows how much air is left in your tank, so you can end the dive with a safe reserve.

Surface intervalPlanning

The time spent at the surface between dives. A longer interval lets more residual nitrogen leave the body, restoring more of your no-decompression limit.

Surface marker buoy (SMB / DSMB)Equipment

An inflatable tube sent to the surface to mark your position for boats — a delayed SMB (DSMB) is deployed from depth before you ascend.

SurgeEnvironment

The back-and-forth water movement from waves, felt most in shallow water. Divers move with it rather than fighting it.

Tank (cylinder)Equipment

The pressurised metal cylinder that holds the compressed air (or nitrox) a diver breathes underwater.

ThermoclineEnvironment

A layer where the water temperature drops noticeably as you descend. Passing through one, the water can suddenly feel much colder.

TideEnvironment

The regular rise and fall of the sea. Tides affect currents, visibility and entry conditions, so dives are often timed around them.

TrimSafety & Skills

Your body position and balance in the water. Good horizontal trim reduces drag, protects the reef and saves air.

Turn pressurePlanning

The tank pressure at which you turn a dive around to head back, planned so you surface with a safe reserve (often about 50 bar / 500 psi).

Valsalva manoeuvrePhysiology

The most common way to equalize: pinch your nose and gently blow against it to push air into the middle ears. Do it early and often, never forcefully.

VertigoPhysiology

A spinning, disoriented feeling underwater, often from one ear equalizing before the other. Pausing, holding a reference and equalizing gently usually settles it.

VisibilityEnvironment

How far you can see underwater, affected by particles, weather and currents. Lower visibility calls for staying closer to your buddy.

Weight check (buoyancy check)Safety & Skills

A quick surface test to confirm you're carrying the right amount of weight — properly weighted, you float at eye level with an empty BCD and a normal breath.

Weight ditchingSafety & Skills

Releasing your weights in an emergency to become positively buoyant and get to the surface. The quick-release is designed for one-handed use.

Weight systemEquipment

Weights worn on a belt or in BCD pockets to offset the natural buoyancy of the body and wetsuit so a diver can descend and hold a safety stop.

WetsuitEquipment

A neoprene suit that keeps a diver warm by trapping a thin layer of water against the skin. It also adds buoyancy, which is offset with weights.