Alternate Air Source (Octopus)
An extra second-stage regulator connected to your first stage. It is usually brightly colored (often yellow) and is used to share air with an out-of-gas diver during an emergency.
ATA (Atmospheres Absolute)
The measurement of total pressure exerted on a diver. At sea level, the pressure is 1 ATA. It increases by 1 ATA for every 10 meters (33 feet) of saltwater descent.
Back Roll Entry
A method of entering the water from a small boat by sitting on the edge of the boat facing inward and gently rolling backward into the water.
Barotrauma
A pressure-related injury caused by unequalized pressure differences between the surrounding water and the body’s air-filled spaces, such as the ears, sinuses, or lungs.
BCD (Buoyancy Control Device)
An inflatable vest or harness worn by the diver. By adding or venting air from the BCD, divers can achieve positive, negative, or neutral buoyancy.
Booties
Neoprene footwear worn to keep feet warm and to prevent blisters when wearing open-heeled fins.
Bottom Time
The total amount of time from the beginning of descent until the diver begins a direct, continuous ascent to the surface or safety stop.
Buddy Check
A pre-dive safety check performed with a dive partner to ensure all equipment is functioning and configured correctly. Often remembered by the acronym BWRAF (Begin With Review And Friend).
Buddy System
The standard safety practice of diving in pairs (or sometimes trios) so that divers can assist one another in an emergency.
CESA (Controlled Emergency Swimming Ascent)
An emergency procedure where an out-of-air diver swims directly to the surface while continuously exhaling to prevent lung over-expansion injuries.
Cylinder / Tank
An aluminum or steel vessel used to store and transport the highly compressed breathing gas (usually air or Nitrox) used by the diver.
DAN (Divers Alert Network)
A global non-profit organization dedicated to dive safety, medical research, and providing emergency medical assistance and insurance for divers.
Decompression Sickness (DCS / “The Bends”)
A potentially dangerous condition caused by nitrogen bubbles forming in the body’s tissues and bloodstream, usually due to a diver ascending too rapidly.
Decompression Stop
A required pause at a specific depth during ascent to allow the body to safely off-gas excess nitrogen. This is mandatory if a diver exceeds their No Decompression Limit (NDL).
Depth Gauge
An instrument that tells the diver their current depth and their maximum depth reached during the dive.
DIN Valve
A type of scuba cylinder valve where the regulator screws directly into the valve itself. Known for forming a highly secure, high-pressure seal.
Dive Computer
A wrist-mounted or console-mounted digital device that monitors depth and time underwater, using an algorithm to calculate safe nitrogen absorption and dictate remaining bottom time.
Dive Flag
A flag used to warn boats that there are divers in the water. In North America, it is a red flag with a white diagonal stripe; internationally, it is the blue and white Alpha flag.
Divemaster
The first level of professional diving certification. Divemasters lead certified divers on underwater tours and assist instructors with teaching.
Drift Dive
A dive where divers let the ocean current carry them along a reef or underwater landscape, rather than swimming against it, usually being picked up by a boat at the end of the dive.
Drysuit
A specialized, watertight suit used for cold-water diving. Air is added to the suit to provide insulation, and divers wear thermal undergarments beneath it.
DSMB (Delayed Surface Marker Buoy)
An inflatable, brightly colored tube that a diver deploys from underwater before ascending. It signals the diver’s position to boats on the surface.
Enriched Air (Nitrox)
A breathing gas blend containing a higher percentage of oxygen than standard air (more than 21%, commonly 32% or 36%). It extends bottom time by reducing nitrogen intake.
Equalization
The physical act of adding air to the body’s air spaces (primarily the middle ear and sinuses) to match the increasing pressure of the surrounding water during descent.
Fins
Blade-like equipment worn on the feet to provide propulsion and maneuverability underwater.
First Stage
The heavy metal part of the regulator that attaches directly to the scuba tank. It reduces the extremely high-pressure air from the tank to an intermediate pressure.
Giant Stride Entry
A standard water entry technique used from a large boat or pier. The diver steps out into the water with one leg extended forward while holding their mask and regulator in place.
Halocline
A distinct, visible, sometimes blurry layer underwater where fresh water and salt water meet but do not immediately mix due to different densities.
Hood
A neoprene head covering worn to trap heat and prevent thermal loss, primarily used in cold water environments.
Hypercapnia
An excessive buildup of carbon dioxide in the bloodstream, often caused by “skip breathing” (holding your breath to save air) or overexertion underwater.
Hypoxia
A condition where there is a dangerous lack of oxygen reaching the body’s tissues.
Liveaboard
A large dive boat designed with sleeping cabins and dining facilities where divers live for several days or weeks, allowing access to remote dive sites.
Logbook
A physical booklet or digital app where divers record the details of their dives, including depth, time, location, and marine life seen, as proof of experience.
Mask
Essential gear that covers the diver’s eyes and nose, creating an air pocket that allows the eyes to focus underwater.
NDL (No Decompression Limit)
The maximum amount of time a diver can stay at a specific depth and still ascend directly to the surface without needing mandatory decompression stops.
Negative Buoyancy
A state where a diver is heavier than the water they displace, causing them to sink toward the bottom.
Neutral Buoyancy
A state of weightlessness underwater where a diver neither sinks nor floats, allowing them to effortlessly hover in the water column.
Nitrogen Narcosis
A reversible alteration in consciousness that occurs at deeper depths (usually below 100 ft/30 m), causing a feeling of intoxication, euphoria, or anxiety due to the narcotic effect of pressurized nitrogen.
O-Ring
A small rubber ring that creates an airtight seal between the tank valve and the first stage of the regulator.
Open Water Certification
The foundational, entry-level certification that allows a diver to rent gear, get air fills, and dive independently with a buddy to a maximum depth of 60 ft (18 m).
Oxygen Toxicity
A dangerous condition resulting from breathing oxygen at too high a partial pressure (usually below safe depth limits for Nitrox or pure O2), which can cause sudden underwater convulsions.
PADI (Professional Association of Diving Instructors)
The world’s largest and most widely recognized recreational scuba diver training organization.
Pony Bottle
A small, fully independent scuba cylinder equipped with its own regulator, carried by a diver as a redundant, self-sufficient backup air supply.
Positive Buoyancy
A state where a diver is lighter than the water they displace, causing them to float toward the surface.
Regulator
The entire mechanism that delivers breathing gas from the tank to the diver at ambient pressure, consisting of a first stage, hoses, and second stages.
Residual Nitrogen
The excess nitrogen remaining dissolved in a diver’s bodily tissues after a dive is completed. It must be accounted for if the diver plans to dive again soon.
Safety Stop
A highly recommended, non-mandatory pause made at 15 feet (5 meters) for 3 minutes during ascent to allow the body to off-gas excess nitrogen before surfacing.
Second Stage
The mouthpiece portion of the regulator that goes into the diver’s mouth. It features a purge button and an exhaust valve, delivering air only when the diver inhales.
Snorkel
A simple plastic or silicone tube allowing a diver to breathe surface air while floating face-down, conserving compressed air from the tank.
SPG (Submersible Pressure Gauge)
A gauge connected to the first stage that displays exactly how much breathing gas is remaining inside the scuba cylinder.
Surface Interval
The amount of time a diver spends on the surface out of the water between two successive dives.
Thermocline
A distinct layer in a body of water where the temperature suddenly and drastically drops between the warmer surface water and the colder deep water.
Trimix
A specialized breathing gas used by technical divers consisting of oxygen, helium, and nitrogen. Helium is added to reduce both nitrogen narcosis and oxygen toxicity on very deep dives.
Visibility (Vis)
The horizontal distance a diver can clearly see underwater. It can range from a few inches in muddy water to over 200 feet in crystal clear oceans.
Weight Belt / Weight System
A nylon belt holding lead blocks, or pouches integrated into a BCD, used to counteract the positive buoyancy of the human body and the wetsuit, allowing the diver to descend.
Wetsuit
A snug-fitting garment made of neoprene foam that provides thermal protection by trapping a thin layer of water against the skin, which the body quickly heats up.
Wreck Dive
A specialized dive focused on navigating or penetrating a sunken ship, aircraft, or artificial reef structure.
Yoke Valve
A traditional type of tank valve connection (also known as an A-clamp) where the regulator slides down over the top of the valve and is clamped tightly into place with a screw.