Underwater science
Absolute pressure
Pressure measured relative to a vacuum, including the atmospheric contribution.
Explore related lessons →PLAIN WORDS FOR AN UNDERWATER WORLD
The diving glossary.
From buoyancy to buddy briefings: 103 terms to help you understand what you’re reading, hearing and learning.
103 definitionsIntroductory edition · Review pending
Underwater science
Air
The atmospheric gas mixture, consisting mainly of nitrogen and oxygen.
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Algorithm
A defined calculation process; dive computers use algorithms to process measurements and settings.
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Ambient light
Light already present in the environment, rather than added by a torch or strobe.
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Backscatter
Light scattered back towards a camera or observer by particles in the water.
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Bar
A unit of pressure equal to 100,000 pascals.
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BCD
Buoyancy control device: equipment with an inflatable chamber used as part of a diver’s buoyancy system.
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Boat dive
A dive accessed from a vessel.
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Boat traffic
Vessel movement in an area that needs consideration in local planning and briefings.
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Boyle’s law
For a fixed amount of gas at constant temperature, pressure and volume vary inversely.
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Breathing gas
Gas intended for breathing during the activity; its composition and suitability must be established.
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Briefing
A discussion that explains the intended activity, arrangements and relevant conditions.
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Buddy
A diving partner participating in an agreed system of communication and mutual awareness.
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Buoyancy
The upward force exerted by a fluid on an immersed object.
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Certification
A qualification awarded after meeting a training programme’s requirements.
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Comfort
A person’s sense of ease with the equipment, environment and activity.
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Communication
Sharing information through agreed words, signals or equipment.
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Configuration
The arrangement and combination of equipment used by a diver.
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Confined water
A pool or water with pool-like conditions used for suitable supervised learning.
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Conservation
Actions intended to protect habitats, species and ecological processes.
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Contingency
An agreed response or alternative for a change from the expected situation.
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Coral
A marine animal; many corals consist of colonies of small polyps.
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Course quote
A provider’s stated price and inclusions for a particular course package.
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Current
Water moving in a particular direction.
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Cylinder
A pressure vessel used to contain a breathing-gas supply.
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Debrief
A discussion after an activity to review observations and identify learning.
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Density
Mass per unit volume.
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Depth
Vertical distance below a reference surface, usually the water surface in recreational diving.
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Displacement
The volume of fluid displaced by an immersed object or its submerged part.
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Dive boots
Footwear used for protection, warmth or compatibility with suitable fins.
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Dive centre
A business or facility offering diving services; its current services need confirmation.
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Dive computer
An instrument that measures dive information and uses settings and models to calculate additional guidance.
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Dive plan
The arrangements developed for a particular dive, its participants and conditions.
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Dive site
A location identified for diving; the name alone does not establish access or suitability.
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Dive torch
A light designed for the underwater environment and its specified operating limits.
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Drysuit
An exposure suit designed to keep most water out; its use requires appropriate instruction.
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Entry
The location or method used to enter the water, determined for the actual conditions and activity.
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Equalisation
Managing pressure differences in air spaces using techniques taught through appropriate instruction.
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Equipment
The items used for a diving activity, selected for the diver and intended environment.
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Equipment hire
Temporary use of equipment supplied by a provider under its rental arrangements.
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Exit
The location or method used to leave the water, determined for the actual conditions and activity.
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Exposure
How open a location is to influences such as wind and swell; the term can also describe a diver’s environmental exposure.
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Exposure protection
Equipment selected to help manage thermal and environmental exposure.
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Fins
Equipment worn on the feet to help move through water.
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First stage
The regulator component connected to the cylinder valve that reduces supply pressure to an intermediate pressure.
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Fit
How equipment matches the wearer’s dimensions and intended use.
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Fitness to dive
Suitability for diving in relation to health and the demands of the activity; individual assessment belongs with appropriate professionals.
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Forecast
A prediction of conditions for a stated area and time.
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Freediving
Breath-hold diving, with its own training and supervision practices.
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Gas volume
The amount of space occupied by a gas under the stated conditions.
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Guided dive
An outing led by a guide under the operator’s arrangements and participant requirements.
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Habitat
The environment in which an organism lives.
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Hand signal
A gesture used to convey an agreed message; its meaning should be confirmed with the group.
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Harness
Straps or a support system that hold equipment on the diver.
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Identification
Determining which organism or group an observation represents, with appropriate attention to uncertainty.
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Instructor
A person qualified and authorised within a training system to teach specified programmes.
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Kelp
A group of large brown algae that can form underwater forests.
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Local knowledge
Experience and information about a place that can help interpret current arrangements and conditions.
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Logbook
A record of diving experiences, factual details and learning reflections.
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Marine debris
Human-made material that has entered the marine environment.
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Marine life
Organisms living in marine environments.
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Marine park
A managed marine area with rules established by its responsible authority.
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Mask
Equipment that creates an air space in front of the eyes for underwater vision.
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Medical screening
A process used to identify health information that may require further assessment before participation.
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Mouthpiece
The part of a snorkel or regulator held in the mouth.
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Neutral buoyancy
A state in which buoyant force balances weight, giving no net upward or downward force from those two forces.
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Nitrox
A nitrogen-and-oxygen breathing mixture; recreational enriched-air nitrox has more oxygen than ordinary air and requires appropriate training.
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Observation
Information recorded by an instrument or person at a particular place and time.
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Open water
A natural-water training or diving setting, distinct from confined-water practice. It also appears in some course names.
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Overhead environment
A setting with a physical obstruction to direct ascent to the surface, such as a cave or wreck interior.
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Polyp
An individual animal body form found in corals and some related organisms.
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Pressure
Force distributed over an area.
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Reef
An underwater structure or raised area that may be formed by rock, coral or other material; not all reefs are coral reefs.
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Refresher
Structured review and practice intended to revisit previously learned knowledge and skills.
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Regulator
Equipment that reduces cylinder gas pressure in stages and supplies breathing gas to the diver.
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Scuba
Underwater diving with a self-contained breathing-gas supply.
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Seagrass
A flowering plant adapted to living submerged in marine water.
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Second stage
The regulator component from which the diver breathes, supplying gas on inhalation in a demand system.
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Servicing
Inspection and maintenance carried out according to the equipment maker’s requirements by an appropriate service provider.
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Shore dive
A dive accessed from land rather than a boat.
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Situational awareness
Attention to relevant people, equipment and environmental changes around an activity.
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Slack water
A period of little tidal current; its local timing is not automatically the time of high or low tide.
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Slate
A surface designed for writing notes underwater with a suitable writing tool.
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Snorkel
A breathing tube used with its upper opening above water; it does not supply gas at depth.
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Sound
Mechanical vibrations travelling through a medium such as air or water.
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SPG
Submersible pressure gauge: an instrument that indicates cylinder pressure.
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Supervision
Oversight under the arrangements of a particular course or activity; its form and scope should be clarified.
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Surface interval
Time spent at the surface between dives.
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Surface marker
A visible surface signalling device. Different designs and deployment methods need appropriate training.
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Swell
Waves that have travelled beyond the area where they were generated.
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Task loading
The demands placed on attention by handling multiple activities or responsibilities.
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Tide
The regular rise and fall of sea level associated mainly with gravitational effects of the Moon and Sun.
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Training agency
An organisation that sets standards and programmes for diving education.
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Trim
The diver’s orientation and balance in the water.
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Underwater photography
Making photographs below the water surface, adding equipment and attention demands to the activity.
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Valve
A device controlling flow; a scuba cylinder valve controls access to its gas supply.
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Visibility
How far a person can see through the water under the observed conditions.
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Water temperature
A measure of how warm or cold the water is at the specified place and time.
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Weight
The force of gravity acting on an object; in everyday diving speech, “weights” also means added ballast.
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Wetsuit
An exposure suit commonly made from insulating neoprene that allows some water inside.
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Wildlife etiquette
Practices intended to let animals behave naturally without disturbance from visitors.
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Wind
Air movement described by speed and direction.
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Zoning
Dividing a managed area into zones with defined purposes and activity rules.
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