Scuba Diving A–Z

A

Alternate air source (Octopus) – A backup second stage used for air-sharing.

Ascent – The controlled return to the surface.

Air consumption (SAC/RMV) – How much gas you use (often measured per minute).

B

Buoyancy – The ability to float, sink, or hover neutrally.

BCD / BC – Buoyancy Control Device (vest) used to control buoyancy.

Buddy system – Diving with a partner for safety and support.

C

Computer (Dive computer) – Device that tracks depth/time and calculates NDL/deco information.

CNS – Central Nervous System oxygen exposure tracking (linked to ppO₂ concepts).

Current – Water movement that can impact effort, gas use, and planning.

D

Decompression (Deco) – Controlled off-gassing during ascent (planned or unplanned).

Deco stop – A planned stop to allow safer nitrogen release.

DPV – Diver Propulsion Vehicle (underwater scooter).

E

EANx / Nitrox – Enriched Air Nitrox (more oxygen than air). See: Nitrox guide.

EAD – Equivalent Air Depth (nitrogen exposure comparison). See: EAD explained.

Equalization – Balancing pressure in ears/mask during descent.

F

Fins – Primary propulsion tool; technique affects gas consumption.

Free-flow – Regulator delivering gas continuously (often from cold/icing or malfunction).

First stage – Regulator stage that reduces tank pressure to intermediate pressure.

G

Gas analysis – Measuring oxygen percentage in nitrox before a dive.

Gauge (SPG) – Submersible Pressure Gauge showing cylinder pressure.

Giant stride – Common water entry method from a boat/platform.

H

Hover – Staying neutrally buoyant without moving up/down.

HP / LP – High-pressure / Low-pressure ports and hoses on first stage.

Hyperventilation – Over-breathing; can increase stress and reduce comfort underwater.

I

Intermediate pressure (IP) – Regulator pressure between first and second stage.

Inflator – Mechanism to add air to BCD (and sometimes drysuit).

In-water recompression – A technical/emergency procedure; not standard recreational practice.

J

J-valve – Older reserve valve system (historical; mostly replaced by SPGs).

Jump entry – Controlled step/jump entry method depending on boat/platform rules.

K

Knot – Speed unit (nautical miles per hour); currents often described in knots.

Kitting up – Preparing and assembling dive equipment before entry.

L

Long hose – Hose configuration used for efficient air-sharing (common in tech/overhead setups).

Lift bag – Inflatable bag for lifting objects or as a surface marker (depending on use).

Leak test – Quick check for leaks after assembling gear.

M

MOD – Maximum Operating Depth (safe depth limit for a gas). See: MOD explained.

Mask – Essential for clear vision; must be equalized during descent.

Multigas – Using multiple gas mixes in one dive (advanced/technical).

N

NDL – No-Decompression Limit (time you can stay without mandatory deco stops).

Nitrogen loading – Absorption of nitrogen into the body during a dive.

Nitrox – Enriched Air (EAN). See: Nitrox guide.

O

O-ring – Small sealing ring used in many connections (valves, fittings, etc.).

Octopus – Alternate air source second stage (buddy breathing/air-sharing).

Overhead environment – Cave/wreck/ice where direct ascent isn’t possible (advanced training required).

P

ppO₂ – Oxygen partial pressure (key for nitrox/MOD). See: ppO₂ made easy.

Partial pressure blending – Method used to mix nitrox/trimix (specialized procedure).

Panicked diver – Emergency scenario; training emphasizes prevention and calm response.

Q

Quick release – Buckles/clips designed for fast removal of gear when needed.

Qualified buddy – Buddy with appropriate training/experience for the dive conditions.

R

Regulator – The breathing system delivering gas from the cylinder to the diver.

Residual nitrogen – Nitrogen remaining from previous dives affecting repetitive dive planning.

ROV – Remotely Operated Vehicle (not a diver tool but used in underwater operations).

S

Safety stop – A short stop near the end of a dive (commonly around 3–5 m) as a safety practice.

SMB / DSMB – Surface Marker Buoy / Delayed SMB for surfacing and boat awareness.

SAC / RMV – Gas usage rate metrics (useful for planning).

T

Trim – Body position in the water; good trim improves efficiency and control.

Trimix – Breathing gas with oxygen, nitrogen, and helium (technical diving).

Thermocline – Layer where water temperature changes rapidly with depth.

U

Underweight / Overweight – Incorrect weighting affects buoyancy and gas use.

Uncontrolled ascent – Rapid ascent; risk increases with poor buoyancy control.

U/W signals – Underwater hand signals used for communication.

V

Valve – Cylinder valve controlling gas flow (open/close/isolator on doubles).

Venting – Releasing air from BCD/drysuit to control buoyancy during ascent.

Visibility – Water clarity; affects navigation and comfort.

W

Weight system – Weight belt or integrated weights to achieve neutral buoyancy.

Wetsuit – Neoprene suit providing thermal protection.

Wreck – Ship/aircraft underwater site; overhead risks may apply.

X

X-deep (brand) – Some “X” terms are brand names; in diving, X is less common as a general glossary letter.

X-axis (trim) – Sometimes used informally to describe body alignment; practical focus is simply “good trim”.

Y

Yoke (A-clamp) – Regulator connection type (common in many recreational setups).

YY valve – Dual outlet valve variant; less common, used in certain configurations.

Z

Zinc anode – Corrosion protection used on boats and some underwater structures.

Zone (training) – Many agencies define training “zones” by depth/experience; always follow your certification limits.