Scuba Diving Glossary: Plain Definitions for Beginners

Equipment

Alternate air source: A spare second stage, often bright yellow, that a buddy can breathe from in an emergency. Also called an octopus.
BCD: Buoyancy control device. A jacket or wing you inflate or vent to control buoyancy. It also holds the cylinder.
Cylinder: The tank that holds compressed breathing gas.
Dive computer: A device that tracks depth and time and estimates your remaining no-stop time.
First stage: The part of the regulator attached to the cylinder valve, which reduces high pressure to an intermediate pressure.
Second stage: The part of the regulator you breathe from, which delivers gas at the surrounding water pressure.
SPG: Submersible pressure gauge. Shows how much gas remains in the cylinder.
SMB: Surface marker buoy. An inflatable tube that shows boats and the surface team where you are.
Wetsuit and drysuit: Exposure suits. A wetsuit lets in a thin layer of water. A drysuit keeps you dry and needs its own training.

Buddy check: A pre-dive check that two divers do on each other's equipment.
Buoyancy: Whether you float, sink or stay level. Neutral buoyancy means hovering without effort.
Equalizing: Adding air to the spaces in your ears, sinuses and mask so their pressure matches the water around you.
Giant stride: A boat or dock entry where you step out into the water.
Backward roll: An entry from a small boat where you sit on the edge and roll back into the water.
Mask clearing: Blowing out water that has entered the mask by breathing out through your nose.
Safety stop: A pause near the end of an ascent, commonly about 3 minutes at about 5 m (15 ft).
Turn pressure: The cylinder pressure, agreed in advance, at which you start heading back.

Absolute pressure: The total pressure on you: air at the surface plus the water above you.
Boyle's law: The rule that a gas shrinks as pressure rises and expands as it falls, at a steady temperature.
Decompression sickness: Illness caused by dissolved gas forming bubbles in the body, usually after too long at depth or too fast an ascent. Needs urgent medical care.
Enriched air (nitrox): A breathing gas with more oxygen than ordinary air. Using it needs specific training.
Nitrogen narcosis: A dulling of judgment caused by nitrogen under pressure, stronger with depth.
No-stop limit: The longest time you can stay at a depth and still ascend directly to the surface without required stops. Also called a no-decompression limit.
Overhead environment: Any place without direct access to the surface, such as a cave or the inside of a wreck. Needs specialized training.
Surface interval: The time spent at the surface between two dives.

C-card: Certification card. Proof that you completed a course.
Confined water: A pool or calm, shallow open water used for first skills.
Open water: A natural body of water, such as the sea, a lake or a quarry, used for training dives.
Referral: Finishing part of a course with one dive center and the rest with another.
Training agency: An organization that writes course standards, trains instructors and issues certifications.