ABSOLUTE PRESSURE The total pressure acting on a body, equal to the sum of atmospheric pressure and gauge pressure.
A-CLAMP FITTING A regulator first stage that attaches to the pillar valve of a tank by means of a yoke that clamps it securely in position.
AIR CONSUMPTION The depletion of oxygen in air through respiration; the rate at which air is used by a diver during the course of a dive.
ALTERNATE AIR SOURCE A redundant source of air supply, in the form of either an octopus second stage, or a completely independent tank and regulator.
AMBIENT PRESSURE The total pressure acting on a body at a given depth. See also absolute pressure.
ANCHOR LINE A rope, cable, or chain that attaches a ship to its anchor.
ANOXIA A medical condition caused by a lack, or severe deficiency of, oxygen in the human body.
AQUA LUNG The first underwater breathing apparatus to use compressed air and a two-stage regulator, designed by Emile Gagnan and Jacques Cousteau.
ARCHIPELAGO A group or chain of islands.
ARTIFICIAL REEF A human-made object deliberately scuttled to create a haven for marine life, and for the enjoyment of divers.
ASCENT Returning from depth to the surface at the end of a dive. This must always be performed carefully to avoid decompression sickness.
ATMOSPHERIC PRESSURE The pressure exerted by the gases in the atmosphere. At sea level, this is equal to 14.7 psi (1 bar).
ATOLL Circular coral reef, often surrounding a lagoon, that has formed around a submerged extinct volcano.
BALLAST Lead weights carried to offset the inherent buoyancy of the diver's exposure suit.
BENDS See decompression sickness
BEZEL A rotatable, notched, or marked outer ring on instruments, such as watches and compasses, used to mark key datum points, such as a bearing.
BOMMIE An isolated outcrop of coral growth on the seabed.
BOYLE'S LAW Physical law stating that at a constant temperature, the volume of a gas is inversely proportional to the pressure exerted on it.
BRACKISH Water that is not as salty as seawater, but has a higher salt content than fresh water.
BREATH-HOLD DIVING See freediving
BUBBLE CHECK A precautionary visual inspection of another diver's gear (conducted underwater) to determine if it has any air leaks.
BUDDY A diver who assumes an informal duty of care over the other member of their assigned "buddy pair" during diving.
BUDDY LINE A safety line for tethering a buddy pair together.
BUOYANCY An upward thrust exerted on an immersed object, which is equal to the weight of the water that has been displaced by the object.
BUOYANCY COMPENSATOR (BC) A jacket that can be inflated and deflated to allow the diver to control their buoyancy.
BUOYANCY CONTROL Manipulation of buoyancy through the use of devices such as a BC, and the breathing cycle.
CAISSON A pressurized, watertight chamber used to undertake construction work underwater.
CERTIFICATION CARD Accreditation proving that a diver has achieved a certain level of dive training with one of the recognized agencies.
CORAL HEAD Protrusions on a coral reef formed by a colony of living coral polyps feeding and growing.
CORAL ISLAND Portion of a coral reef that is permanently out of water, its surface usually eroded to a flat top of white coral rock and sand. May occur in a chain along a reef.
CORAL REEF A massive, marine, rocklike ridge or outcrop created by the gradual accretion of the skeletons of generations of coral polyps.
DATUM LINE A line used as a fixed reference to aid in an accurate survey of an area.
DECOMPRESSION Returning to conditions of normal atmospheric pressure by controlled means.
DECOMPRESSION SCHEDULE A plan describing the number, duration, and depth of decompression stops required for a given dive.
DECOMPRESSION SICKNESS (DCS) A potentially dangerous medical condition resulting from the formation of bubbles of nitrogen in the bloodstream and tissues of a diver's body. Also known as decompression injury (DCI) and the bends.
DECOMPRESSION STOP A scheduled pause in a diver's ascent to allow nitrogen to pass from body tissues back into the blood at a safe rate.
DEHYDRATION A medical condition resulting from excessive loss of water from the body.
DELAYED SURFACE MARKER BUOY (DSMB) Marker buoy deployed just before ascent. It is inflated underwater, then released to indicate where a diver will surface.
DEMAND VALVE See second stage
DEPTH GAUGE An analog or digital gauge that measures ambient pressure, and uses this to give a reading of depth.
DESCENT Traveling from the surface to depth.
DIN FITTING A screw-thread fitting for attaching a regulatorfirst stage to a compatible pillar valve. Allows higher working pressures than A-clamp fittings.
DISPLACEMENT The displacement of water that occurs when an object is submerged in it.
DISTANCE LINE A line fastened to a submerged feature and deployed from a reel as a navigational guide, especially in areas of low visibility.
DIVE CENTER A commercially run diving school and rental center, frequently offering organized trips and guides to local dive sites.
DIVE COMPUTER A digital device that provides the diver with a range of information, such as depth, time, and ascent rate.
DIVE CONSOLE An instrument panel incorporating a tank contents gauge, depth gauge, and sometimes also a compass, or a dive computer.
DIVE GUIDE An employee of a dive center or school who guides qualified divers around localdive sites.
DIVE LEADER The diver in a buddy pair who is designated (by agreement) as the principle pacesetter and decision-maker underwater.
DIVE MARSHAL A senior diver whose role on a dive trip is to allocate buddy pairs, approve dive plans, ensure that complete records of dives are kept, and initiate and direct rescue efforts in the event of a diving accident.
DIVE PLAN The outline of a proposed dive, including its depth, duration, intended goals, entry and exit points, signaling conventions, and contingency plans.
DIVE SITE Any location where diving is conducted.
DIVE SLATE A plastic tablet used for making notes underwater.
DIVE TIME The duration of a dive, starting at the moment the diver leaves the surface and ending with their return to the surface.
DIVEMASTER A person qualified to act as a dive guide, assist diving instructors, and oversee the organization of dives for paying customers.
DIVING CLUB An organization offering diving instruction and social events on an amateur basis. It can also refer to continental European diving centers offering instruction on a commercial basis.
DIVING SCHOOL A center for instruction in diving skills (usually on a commercial basis).
DPV (DIVER PROPULSION VEHICLE) A propulsion device either ridden or clung to by a diver to minimize swimming effort or aid in the exploration of large dive sites.
DRAG Frictional resistance to the motion of an object through a fluid.
DRIFT DIVE A dive in which the diver is propelled along a route by undersea currents.
D-RING A D-shaped ring of metal or plastic used as an anchor point for tethering equipment to your BC during a dive.
DROP-OFF A falling-away of the ground or seabed, such as a reef wall or rock shelf that descends steeply.
DRYSUIT An exposure suit designed for use in very cold waters, in which trapped air is the principle insulating medium, and that offers a high degree of thermal protection.
DSMB (DELAYED SURFACE MARKER BUOY) A tubular buoy designed to be deployed at the end of a dive to aid ascent and alert observers at the surface.
DUMP VALVE One-way valve used to release air from an inflatable device.
DYE MARKERS A fluorescent dye designed to be released into the water to alert rescue craft to the position of a drifting diver.
ECOSYSTEM The interactions between a community of living organisms and the environment they inhabit.
EL NINO A warming of the eastern tropical Pacific occurring every few years that has a pronounced effect on both the local and wider climate. Often occurs around Christmas time— "El Nino" in Spanish refers to the Christ child.
EQUALIZATION The process of "clearing" the ears and sinuses until the pressure within them matches the pressure of the water surrounding the body. See also Valsalva maneuver.
EXPOSURE SUIT Any suit worn by a diver in order to prevent excessive loss of body heat. See wetsuits, semi-dry suits, and drysuits.
FILTER-FEEDER An aquatic animal that feeds by straining minute organisms and other suspended food particles from the water.
FINNING Kicking the feet rhythmically while wearing fins to achieve forward motion.
FIRST STAGE A major component of the regulator that reduces the pressure of air from the tank to 8—10 bar above ambient pressure, then allows it to pass to the second stage for inhalation by the diver. It also supplies air for inflatable items such as the BC, and to pressure gauges displaying the remaining contents of the tank.
FOOD CHAIN A sequence of organisms, each depending on the next as a source of food.
FOOT POCKET A style of fin shoe that covers the whole foot.
FREE ASCENT An ascent that is carried out without the use of a line or any other point of reference.
FREEDIVING A form of diving in which divers do not carry an air supply but remain underwater only as long as they can hold their breath.
GAUGE PRESSURE The surrounding water pressure, as measured by a gauge that does not take atmospheric pressure into account.
GLOBAL POSITIONING SYSTEM (GPS) A navigation system that uses satellite signals to determine an exact longitude and latitude position.
GUIDELINE A line with a weight at one end and a buoy at the other, chiefly used to mark the position of wrecks and provide a guide for descending and ascending divers.
HELIOX A mixture of helium and oxygen, used for diving beyond recreational diving limits.
HYDROSTATIC TESTING A method of testing a tank for serviceability by filling it with pressurized water.
HYPERVENTILATION A medical condition caused by rapid or deep breathing, resulting in lowered carbon dioxide levels in the blood.
HYPOXIA A medical condition caused by a deficiency of oxygen in the human body.
INFLATOR VALVE A manually operated valve that admits compressed air into the buoyancy bladders of a BC, or into a drysuit.
KELP FOREST A marine ecosystem based around dense growths of seaweed belonging to the kelp family.
LIVEABOARD A recreational diving vessel equipped to conduct extended diving trips without returning to port.
LOG BOOK A record containing the details of individual dives made by a diver.
LONGSHORE CURRENT A current running parallel to the shoreline.
MANGROVE Assorted coastal tree and shrub species with aerial roots that grow in dense thickets along tidal shores in the tropics.
MARINE PARK An area of sea or coastline in which marine species are protected by government legislation.
MASK FOGGING Misting caused by the accumulation of oils on the lens (or lenses) of a diving mask, permitting moisture to condense there more readily.
MASK SQUEEZE Discomfort caused by increasing water pressure acting on the diver's mask, pressing its frame into the face.
MUCK DIVING Informal term for diving in areas where the bottom sediments are dark and coarse but where small marine species proliferate (such as Sulawesi's Lembeh Straits).
NEAP TIDE The point in the monthly cycle of tides where the gravitational influences of the Sun and Moon are least aligned, resulting in the lowest high tides and the highest low tides.
NEOPRENE A synthetic rubber fabric with good heat insulation properties used to make wetsuits.
NITROGEN NARCOSIS Intoxication caused by breathing nitrogen at elevated partial pressures (such as during deep dives), resulting in the impairment of reasoning and motor skills, feelings of irrational anxiety or elation, and perceptual narrowing.
NITROX Any mixture of nitrogen and oxygen prepared for use as a breathing gas, but normally one containing a greater percentage of oxygen than that found in air.
OCTOPUS SECOND STAGE A redundant second stage, carried by divers as a contingency against failure of the main second stage, and to enable air-sharing with a diver whose own air supply has been depleted.
OFF-GASSING The gradual release of gases (especially nitrogen) that have accumulated in a diver's body during the course of a dive. Off- gassing occurs as ambient pressure decreases during ascent and at the surface.
OPEN WATER A body of water that is not sheltered from the influence of naturally developing weather conditions and currents, and whose depth may exceed safe training limits.
O-RING A ring-shaped gasket, used to seal interfaces between one pressurized piece of equipment and another, such as in the pillar valve of a tank.
OVERHANG A projection from the face of a reef, cliff, etc.
OVERHEAD ENVIRONMENT Any diving environment where direct access to the surface is restricted, such as inside a cave or under ice.
OXYGEN TOXICITY Acute nervous system dysfunction caused by breathing oxygen at elevated pressures. Can cause loss of consciousness and convulsions.
PARTIAL PRESSURE The pressure of a single gas in a mixture of gases, measured as if that gas alone were present (the total pressure of a gas mixture being the sum of the partial pressures of its constituent gases).
PELAGIC ANIMALS Creatures that dwell in the open ocean, rather than coastal waters.
PILLAR VALVE The valve that transfers gas from the diver's tank to the regulator first stage.
PLANKTON Minute free-floating plant and animal organisms inhabiting seas and lakes, which form the basis of the food chain in many environments.
PLB (personal locator beacon) A submersible device that emits a radio signal on an internationally recognized search-and-rescue homing frequency, enabling emergency services to pinpoint its location.
POLAR SEAS The cold seas at the polar extremities of the planet, which vary in temperature from 29°F to 50°F (-1.8°C to 10°C).
PONY BOTTLE A small tank of breathing gas with an independent regulator, intended for emergency use if the diver's main breathing apparatus fails.
PRESSURE The exertion of force by one surface, substance, or gas onto another.
REBREATHER A breathing apparatus that recycles exhaled gas by filtering out carbon dioxide and replacing any oxygen metabolized by the diver.
RECOMPRESSION CHAMBER A pressure chamber used in the treatment of divers suffering from decompression sickness.
RECREATIONAL DIVING Sports diving that takes place within certain depth and environmental limitations, as defined by the various training agencies.
REEF WALL A vertical face of coral growth, often descending into deep waters.
REEL A spooling device used to store and deploy line.
REFRACTION The change in the velocity of light (or any propagating wave) that occurs when it passes from one medium to another (e.g., from air to water).
REGULATOR The mechanism that supplies breathing gas from the tank to the diver via a two-stage process of pressure reduction. The regulator is made up of two parts: the first stage and the second stage.
RIB (rigid inflatable boat) A fast utility boat characterized by a V-shaped hull with an inflatable tube running around its upper edge.
RIP CURRENT A fast-moving current running perpendicular to the shore, caused by the action of incoming waves.
RULE OF THIRDS, THE A rule of thumb used when calculating air requirements for a dive, especially one requiring a conservative approach, such as a cavern exploration.
SAFETY SAUSAGE See SMB
SAFETY STOP A precautionary pause in a diver's ascent to prevent the formation of nitrogen bubbles in the bloodstream. Its purpose is as a failsafe against decompression sickness even when no scheduled decompression stops need to be made.
SALINITY The salt content of a measure of water.
SCUBA Acronym for "self-contained underwater breathing apparatus."
SECOND STAGE The part of the regulator that regulates the pressure of a diver's gas supply to the same pressure as its immediate environment.
SEMI-DRY SUIT A thick neoprene exposure suit, similar to a wetsuit, designed for use in cold waters.
SHELTERED WATER A shallow body of water protected from currents and weather conditions, used for training divers in a controlled environment.
SILTING A rapid reduction in underwater visibility that occurs when fine sedimentary particles are stirred up.
SKINSUIT A thin exposure suit, typically made of Lycra, designed for use in tropical waters.
SLACK WATER A period of reduced water movement that occurs between incoming and outgoing tides.
SMB (surface marker buoy) An inflatable buoy designed to be towed behind a diver to indicate their position to observers on the surface. Long, cylinder-shaped SMBs are known as safety sausages.
SNORKELING Swimming face-down at the surface using a short, curved tube to draw breath.
SPG (SUBMERSIBLE PRESSURE GAUGE)See tank contents gauge
SPRING TIDE The point in the monthly cycle of tides when the highest high tides, and lowest low tides occur. Spring tides occur when the gravitational forces of the Sun and the Moon are aligned.
SPUR AND GROOVE REEF FORMATION - A typical growth structure of coral reefs, in which outcrops of coral are separated by linear depressions.
STANDING CURRENT A continuous directional movement of water generated by prevailing weather, sea, and climatic conditions.
STROBE A beacon that emits high-intensity flashes of light at short intervals.
SUBTROPICAL SEAS Warm seas within a loosely defined zone between temperate and tropical climatic conditions.
SWELL The undulating motion of surface waters in areas of open sea.
SWIM-THROUGH A submerged arch, or short passage of rock, where direct access to the surface is restricted, but in which the exit is clearly visible at all times.
TANK A steel or aluminum container designed to hold compressed gas.
TANK CONTENTS GAUGE An instrument that displays the remaining quantity of air or other breathing gas in a diver's tank. Also known as a submersible pressure gauge (SPG) or pressure gauge.
TECHNICAL DIVING The use of special breathing-gas mixtures, such as Nitrox, Trimix, and Heliox, to extend depth limits and dive durations beyond recreational diving limits.
TEMPERATE SEAS Seas of an intermediate temperature between tropical and polar, typically 50-68°F (10-20°C).
TEST DATE The date when a compressed gas tank was last subjected to hydrostatic testing.
THERMOCLINE A layer in a body of water in which temperature changes rapidly with depth. This is usually between a warm surface layer and colder deep water.
TIDAL CURRENT A current generated by the action of the tides.
TIDAL CYCLE The cyclic rise and fall of shoreline sea level, caused by the gravitational influence of the Moon and Sun.
TOPOGRAPHY The shape or form of a region of land or seabed.
TRIMIX A mixture of oxygen, helium, and nitrogen commonly used as a breathing gas for dives deeper than 165 ft (50 m).
TROPICAL SEAS The seas found within the boundaries of the 68°F (20°C) isotherm—a band of warm equatorial water.
VALSALVA MANEUVER A method of manually increasing pressure on the inside of the eardrum by closing the nasal passage and exerting expiratory effort, forcing air through the eustachian tubes into the inner ear.
WATER COLUMN The zone of water between the surface and the bottom of a lake, sea, or river.
WEIGHT SYSTEM The means by which a diver's lead ballast is secured.
WETSUIT An exposure suit made of neoprene rubber, which traps a thin layer of water against the skin as an insulating medium.
WICKING The absorption of moisture into a fabric by capillary action.
WORKING PRESSURE The normal pressure at which an equipment item is designed to operate.
WORLD HERITAGE SITE An area of natural or cultural importance protected by a United Nations treaty.
WRECK Remains of ships, aircraft, or other vehicles found on the seabed.