Dive gear comes with its own vocabulary, and product pages assume you already know it. This glossary explains 25 common terms in plain English so that specifications and buying guides are easier to follow. The definitions are general and neutral. They describe how the words are normally used in diving, not how a single brand uses them.
The Scuba Forge Editorial Team uses these terms throughout its guides, and they are listed in alphabetical order. A glossary is no substitute for training. If you are new to diving, a qualified instructor is the right place to learn what these words mean in practice. If you think a definition needs fixing, write to editorial@scubaforge.com.
- Air consumption
- Air consumption is how quickly a diver uses the gas in a tank. It varies with depth, effort, water temperature and experience. Divers often express it as a surface air consumption rate.
- Ambient pressure
- Ambient pressure is the pressure of the surroundings, which rises with depth underwater. It is the sum of atmospheric pressure and the weight of the water above. Regulators deliver breathing gas at the ambient pressure.
- BCD
- A BCD, or buoyancy control device, is a vest or wing that holds air to adjust how much a diver floats. Adding air makes the diver more buoyant, and releasing it makes the diver less buoyant. Many models also carry the tank and hold pockets.
- Bottom time
- Bottom time is the time spent underwater from leaving the surface until starting the final ascent. Dive planning tools use it together with depth to estimate nitrogen loading. Definitions can differ slightly between training agencies.
- Buoyancy
- Buoyancy is the upward force that water exerts on a diver. A diver who floats is positively buoyant, one who sinks is negatively buoyant, and one who hovers is neutrally buoyant. Weights and a BCD are used to control it.
- Decompression sickness
- Decompression sickness is a medical condition caused by dissolved gas forming bubbles in the body after a pressure drop, often following a fast ascent. Divers manage the risk with ascent rates, limits and stops. It requires medical attention.
- Depth gauge
- A depth gauge shows how deep a diver is. It can be a mechanical gauge or part of a dive computer. Accuracy and display style vary between models.
- Dive computer
- A dive computer is a wrist or console device that tracks depth, time and a calculated gas exposure. It displays limits and ascent information using a decompression algorithm. Features vary widely between models.
- Equalizing
- Equalizing means balancing the air pressure in the ears and sinuses with the surrounding water pressure as a diver descends. Techniques include pinching the nose and gently blowing. Some masks with nose pockets make this easier.
- Fin
- A fin is a blade worn on the foot that increases the thrust from each kick. Types include paddle fins and split fins, and the heel can be an open or a full-foot style. Stiffness and length affect how they feel to use.
- First stage
- The first stage of a regulator attaches to the tank valve and reduces the high tank pressure to an intermediate pressure. A hose then carries that gas to the second stage. Designs include piston and diaphragm types.
- Free flow
- A free flow happens when a regulator releases gas continuously instead of only when the diver inhales. It can be caused by cold water, a fault or poor servicing. Divers are trained to respond to it.
- Lumen
- A lumen is a unit of the total light a source emits. Dive light makers often use it to describe brightness. The beam angle and the way light is focused affect how bright it looks at a distance.
- Neutral buoyancy
- Neutral buoyancy is the state of neither sinking nor floating, so a diver can hover at a chosen depth. Divers adjust it with a BCD, weights and breathing. It also helps protect reefs and saves energy.
- No-decompression limit
- The no-decompression limit is the longest time a diver can stay at a given depth and still ascend directly to the surface without required stops, according to a dive table or computer. It shortens as depth increases. Algorithms differ, so values differ.
- Octopus
- An octopus is a spare second stage on a separate hose, kept ready for a buddy to use in an emergency. It is usually a bright color so it is easy to find. Some divers prefer a similar alternate source arrangement.
- Purge button
- The purge button on a second stage manually releases gas to clear water from the mouthpiece. It can also help in checking that the regulator delivers air. It should be pressed only briefly.
- Regulator
- A regulator reduces high-pressure gas in a tank to a pressure a diver can breathe. It usually has a first stage, a second stage and often a spare second stage and gauges. It is life-support equipment and needs regular servicing as the maker directs.
- Safety stop
- A safety stop is a short pause, often a few minutes at a shallow depth, made near the end of an ascent. Many training agencies teach it as a precaution. Dive computers may show a countdown for it.
- Second stage
- The second stage is the part of a regulator that goes in the mouth. It reduces intermediate pressure to ambient pressure and gives gas on each breath. It includes the mouthpiece, a purge button and an exhaust.
- Snorkel
- A snorkel is a tube that lets a person breathe at the surface with the face in the water. Many divers carry one for surface swimming. Some have a purge valve at the bottom to clear water.
- Submersible pressure gauge
- A submersible pressure gauge, or SPG, shows how much gas is left in the tank. It connects to the first stage by a high-pressure hose. Some dive computers show the same information wirelessly.
- Tempered glass
- Tempered glass is glass treated to resist impact, and it is the usual lens material in quality dive masks. It breaks into small pieces rather than sharp shards. Plastic lenses are lighter but scratch more easily.
- Trim
- Trim describes a diver’s body position in the water, ideally horizontal and streamlined. Good trim lowers drag and lowers air use. Weight placement on the body affects it.
- Wetsuit
- A wetsuit is a neoprene suit that lets a thin layer of water warm up against the skin. Thickness is measured in millimeters, and thicker suits keep a diver warmer. Compression reduces insulation at depth.