Scuba diving glossary

Every term you’ll meet on the Open Water exam, explained in plain language. No jargon, no rote memorising — just clear definitions that make the theory click.

Air consumption (SAC rate): 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.
Ascent rate: 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.
Barotrauma: 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): The inflatable jacket a diver adds air to or vents to float, sink or hover. It's the main tool for controlling buoyancy underwater.
Boyle's Law: 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 system: Diving in pairs so each diver can help the other — sharing air, watching for problems and assisting in an emergency. A core safety practice.
Buoyancy: The tendency to float or sink in water. Divers aim for neutral buoyancy, where they neither rise nor sink and can hover effortlessly.
Decompression sickness (the bends): 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.
Dive computer: A wrist or console device that tracks your depth and time and continuously calculates your remaining no-decompression limit, ascent rate and safety stop.
Equalization: Gently adding air to the middle ears (and other air spaces) on descent to balance the rising water pressure and prevent pain or injury.
Mask squeeze: A pressure imbalance inside the mask on descent. Divers prevent it by exhaling a little air through the nose into the mask.
Nitrogen narcosis: 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): 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): 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.
Octopus (alternate air source): A spare second-stage regulator a diver carries so a buddy can breathe from the same tank in an out-of-air emergency.
Partial pressure: 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.
Regulator: The device that reduces high-pressure tank air to a breathable pressure and delivers it on demand each time you inhale.
Residual nitrogen: 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.
Safety stop: 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.
SPG (submersible pressure gauge): The gauge that shows how much air is left in your tank, so you can end the dive with a safe reserve.
Surface interval: 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.
Tank (cylinder): The pressurised metal cylinder that holds the compressed air (or nitrox) a diver breathes underwater.
Weight system: 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.
Wetsuit: 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.