The Diver's Dictionary

100 essential scuba diving and dive computer terms — from decompression theory to gear, gas management, and safety protocols.

ABCDEFGHILMNOPRSTVW

A

Absolute PressureAbsolute pressure is the total pressure at a given point, including atmospheric pressure. It is the sum of atmospheric…
Advanced Open WaterAdvanced Open Water Diver is the second-level recreational scuba certification that extends the depth limit to 30…
AirIn scuba diving, air refers to the standard breathing gas composition of approximately 21% oxygen and 79% nitrogen. It…
Air IntegrationAir integration is a dive computer feature that tracks your remaining tank pressure in real time, either through a…
AlgorithmIn diving, an algorithm is the mathematical model a dive computer uses to calculate nitrogen (and sometimes helium)…
Alternate Air SourceAn alternate air source is any backup device that provides breathable air to a diver or their buddy in an emergency.…
Ambient PressureAmbient pressure is the total pressure exerted on a diver by the surrounding environment — the combined weight of the…
Ascent RateAscent rate is the speed at which a diver travels upward during the ascent phase of a dive. The universally accepted…
ATM / ATAATA (atmospheres absolute) is a pressure unit used extensively in decompression theory and dive physics. One ATA equals…
Audible AlarmAn audible alarm is a piezoelectric buzzer or speaker in a dive computer that produces warning tones when critical…

B

BacklightA backlight is the illumination system behind or around a dive computer's display that makes the screen readable in…
Bailout GasBailout gas is a redundant breathing gas supply carried by a technical or rebreather diver as an emergency backup. If…
BarBar is a metric unit of pressure widely used in diving. One bar is approximately equal to atmospheric pressure at sea…
BarotraumaBarotrauma is physical tissue damage caused by pressure changes affecting gas-filled spaces in the body. As a diver…
Battery TypeDive computers use various battery types depending on their design and power requirements. Common options include…
BCD (Buoyancy Compensator)A BCD (buoyancy compensator device, also called a buoyancy control device) is an inflatable vest or wing worn by scuba…
Bluetooth SyncBluetooth sync is a wireless data transfer feature that allows dive computers to connect to a smartphone or tablet to…
BootiesDive booties are neoprene footwear worn inside open-heel fins to provide thermal insulation, protect feet from sharp…
Bottom TimeBottom time is the duration of the underwater portion of a dive. Different agencies and dive computers define the…
Boyle's LawBoyle's law states that at a constant temperature, the volume of a gas is inversely proportional to its pressure. In…
Breathing RateBreathing rate in diving refers to how much gas a diver consumes per minute, typically measured as respiratory minute…
Buddy SystemThe buddy system is the standard practice of diving with a partner who can provide mutual assistance, air sharing, and…
Bühlmann AlgorithmThe Bühlmann algorithm, formally known as ZHL-16C, is the most widely used decompression model in dive computers.…

C

Compass BearingA compass bearing is a directional heading measured in degrees from magnetic north (0° to 360°) that divers use for…
Conservatism SettingA conservatism setting is a user-adjustable parameter on a dive computer that adds an extra safety margin to the…
ConsoleA console is a multi-instrument housing connected to the first stage via a high-pressure hose that typically combines…
Controlled Emergency Swimming Ascent (CESA)A controlled emergency swimming ascent (CESA) is an emergency procedure used when a diver runs out of air and cannot…
CurrentA current is the horizontal movement of water caused by tides, wind, temperature differentials, or ocean circulation…

D

Dalton's LawDalton's law states that the total pressure of a gas mixture equals the sum of the partial pressures of each individual…
DAN (Divers Alert Network)DAN (Divers Alert Network) is a nonprofit organization dedicated to dive safety through medical research, emergency…
Decompression Sickness (DCS)Decompression sickness (DCS), commonly called "the bends," is a condition caused by nitrogen bubbles forming in the…
Decompression StopA decompression stop is a mandatory pause at a specific depth during ascent where a diver must remain for a calculated…
Digital CompassA digital compass is an electronic magnetometer integrated into a dive computer that displays directional heading on…
Dive Accident ManagementDive accident management encompasses the procedures for recognizing, responding to, and managing diving-related…
Dive ComputerA dive computer is a wrist-mounted or console-mounted device that continuously tracks depth, time, water temperature,…
Dive FlagA dive flag is a surface marker that warns boat traffic of divers below. Two styles are in common use: the red flag…
Dive KnifeA dive knife is a cutting tool carried by scuba divers as a safety device for freeing themselves from entanglement in…
Dive LightA dive light is a waterproof flashlight or torch designed for underwater use. Primary dive lights provide wide,…
Dive LogA dive log is a record of a completed dive stored in your dive computer's memory. Each log entry typically includes…
Dive PlanA dive plan is the pre-dive strategy covering all aspects of an upcoming dive: maximum depth, planned bottom time, gas…
Dive SiteA dive site is a specific underwater location selected for diving based on its features — coral reefs, wrecks, walls,…
DivemasterDivemaster is the first professional-level certification in recreational scuba diving. Divemasters are authorized to…
DrysuitA drysuit is a sealed diving garment that keeps the diver completely dry by using waterproof seals at the wrists, neck,…

E

Enriched Air Nitrox (EANx)Enriched air nitrox (EANx) is a breathing gas with a higher oxygen percentage than standard air, typically 32% (EAN32)…
EntanglementEntanglement is a hazard where a diver becomes caught in fishing line, nets, kelp, rope, cable, or other material that…
Equivalent Narcotic Depth (END)Equivalent narcotic depth (END) is a way to estimate the narcotic effect of a gas mix at depth by expressing it as the…

F

FinsDive fins are blade or split-blade attachments worn on the feet that convert leg kicks into forward propulsion…
Firmware UpdateA firmware update is a software upgrade pushed to a dive computer's internal processor to fix bugs, add features, or…
First StageThe first stage is the component of a scuba regulator that attaches directly to the tank valve and reduces the…
Free FlowA free flow is a regulator malfunction where the second stage delivers air continuously rather than only on demand. The…

G

Gas SwitchingGas switching is the practice of changing between different breathing gas mixes during a single dive, most commonly…
Gauge PressureGauge pressure is pressure measured relative to atmospheric pressure — it reads zero at the surface and only measures…
GlovesDiving gloves are neoprene or textile hand coverings that provide thermal protection, abrasion resistance, and grip…
Gradient FactorGradient factors (GF) are a pair of percentages that control how conservatively a Bühlmann algorithm manages…

H

Half-TimeHalf-time is the time required for a tissue compartment to absorb or release half the difference between its current…
HaloclineA halocline is a water layer where salinity changes sharply — typically where freshwater meets saltwater, such as in…
HelioxHeliox is a breathing gas mixture containing only helium and oxygen, with no nitrogen. Used primarily in commercial and…
Henry's LawHenry's law states that the amount of gas that dissolves in a liquid is proportional to the partial pressure of that…
HoodA diving hood is a neoprene head covering that reduces heat loss from the head and protects the ears and neck in cold…

I

InstructorA scuba Instructor is a fully certified dive professional authorized to teach independent scuba courses, certify new…

L

LiveaboardA liveaboard is a vessel — typically a yacht, catamaran, or purpose-built dive boat — on which divers live aboard for…
Lost Buddy ProcedureA lost buddy procedure is the standardized steps to follow when you lose visual contact with your dive buddy…
Lung OverexpansionLung overexpansion injury occurs when a diver ascends while holding their breath, causing the expanding air in the…

M

M-ValueAn M-value (Maximum Value) is the maximum tissue gas tension that a decompression algorithm allows in a specific tissue…
MaskA dive mask is a sealed lens and skirt assembly that creates an air pocket in front of the eyes, allowing underwater…
Maximum Operating Depth (MOD)Maximum operating depth (MOD) is the deepest depth at which a specific breathing gas can be safely used, based on the…

N

Nitrogen NarcosisNitrogen narcosis is a reversible alteration in consciousness caused by breathing nitrogen at elevated partial…
No-Decompression Limit (NDL)The no-decompression limit (NDL) is the maximum amount of time you can spend at a given depth without being required to…

O

OctopusAn octopus is the backup second-stage regulator carried by scuba divers as an alternate air source for a buddy in an…
Open Water DiverOpen Water Diver is the entry-level scuba certification that qualifies a diver to dive independently (with a buddy) to…
Out-of-Air EmergencyAn out-of-air emergency occurs when a diver's air supply is depleted or their regulator fails, cutting off their…
Oxygen Percentage (FO2)Oxygen percentage, technically called the fraction of oxygen (FO2), is the proportion of oxygen in a breathing gas,…
Oxygen ToxicityOxygen toxicity is a potentially life-threatening condition caused by breathing oxygen at partial pressures above safe…

P

Partial Pressure (ppO2)Partial pressure is the pressure exerted by a single gas in a mixture, calculated by multiplying the fraction of that…
PSIPSI (pounds per square inch) is an imperial unit of pressure used primarily in North American diving for tank pressure…

R

Rash GuardA rash guard is a lightweight, quick-drying shirt made from lycra, spandex, or similar synthetic material worn under a…
Recreational DivingRecreational diving is scuba diving within established limits designed to maintain a wide safety margin for trained…
ReelA dive reel is a spool of line used for deploying surface marker buoys, laying guide lines in overhead environments…
RegulatorA scuba regulator is the system that reduces high-pressure air from the tank to a breathable pressure. It consists of a…
Repetitive DiveA repetitive dive is any dive that begins before your body has completely off-gassed the residual nitrogen from a…
Rescue DiverRescue Diver is a certification focused on stress management, emergency recognition, and rescue skills. It teaches…
RGBMRGBM (Reduced Gradient Bubble Model) is a decompression algorithm developed by Dr. Bruce Wienke that models both…

S

Safety StopA safety stop is a voluntary 3-minute pause at approximately 5 meters (15 feet) during ascent on a no-decompression…
Second StageThe second stage is the mouthpiece portion of a scuba regulator that delivers breathing air on demand. When you inhale,…
SMB (Surface Marker Buoy)An SMB (surface marker buoy) is an inflatable, brightly colored tube that a diver deploys from depth to mark their…
SnorkelA snorkel is a breathing tube that allows a diver to breathe face-down at the surface without lifting the head or using…
SPG (Submersible Pressure Gauge)An SPG (submersible pressure gauge) is a mechanical gauge connected to a high-pressure port on the regulator's first…
SqueezeSqueeze is the compression of a gas-filled body space or equipment space caused by increasing water pressure during…
Surface Air Consumption (SAC)Surface air consumption (SAC) rate is a standardized measure of a diver's gas consumption expressed as the volume of…
Surface IntervalA surface interval (SI) is the time spent out of the water between dives. During this period, your body continues to…

T

Technical DivingTechnical diving (tec diving) is diving that exceeds recreational limits — deeper than 40 meters, requiring…
ThermoclineA thermocline is a distinct layer of water where the temperature changes rapidly with depth — often a drop of 3 to 10°C…
Tissue CompartmentA tissue compartment is a mathematical construct in decompression algorithms that represents a group of body tissues…
TrimixTrimix is a breathing gas mixture containing oxygen, nitrogen, and helium, used primarily in technical diving to reduce…

V

VisibilityVisibility (vis) is the horizontal distance a diver can see underwater, measured in meters or feet. It is one of the…
Visual AlarmA visual alarm is an on-screen alert displayed by a dive computer when a safety threshold is triggered. Visual alarms…
VPMVPM (Varying Permeability Model) is a bubble-based decompression algorithm that focuses on the physics of gas bubble…

W

Watch ModeWatch mode is a function on wrist-mounted dive computers that displays the current time, date, and sometimes additional…
WetsuitA wetsuit is a form-fitting neoprene garment that provides thermal insulation for scuba divers by trapping a thin layer…
Wireless TransmitterA wireless transmitter is a small, battery-powered device that threads into a high-pressure port on your regulator's…
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