| Term | Explanation |
|---|
| Altitude dive | A dive made at an elevation greater than 300 m (1000 ft) above sea level. |
| Ascent rate | The speed at which the diver ascends toward the surface. |
| Ascent time | The minimum amount of time needed to reach the surface on a decompression dive. |
| CCR | Closed-Circuit Rebreather. Scuba that recycles all exhaled gas. |
| Ceiling | On a decompression dive, the shallowest depth to which a diver may ascend based on computed inert gas load. |
| CNS | Central nervous system toxicity. Toxicity is caused by oxygen. Can cause a variety of neurological symptoms. The most important of which is an epileptic-like convulsion which can cause a diver to drown. |
| CNS% | Central nervous system toxicity limit fraction. |
| Compartment | See Tissue group. |
| DCS | Decompression sickness/illness. Any of a variety of maladies resulting either directly or indirectly from the formation of nitrogen or helium bubbles in tissues or body fluids, as a result of inadequately controlled decompression. |
| Decompression | Time spent at a decompression stop, or range, before surfacing, to allow absorbed nitrogen to escape naturally from tissues. |
| Decompression window | On a decompression dive, the depth range between the floor and the ceiling within which a diver must stop for some time during ascent. |
| Dive series | A group of repetitive dives between which the dive computer indicates some nitrogen loading is present. When nitrogen loading reaches zero the dive computer deactivates. |
| Dive time | Elapsed time between leaving the surface to descend, and returning to the surface at the end of a dive. |
| END | Equivalent Narcotic Depth is used as a way of estimating the narcotic effect of a breathing gas typically with trimix gas mixtures. For a given breathing gas mixture and depth the END indicates the depth which would produce the same narcotic effect when breathing compressed air. |
| Floor | The deepest depth during a decompression dive where it is recommended to stop for an effective decompression. |
| He% | Helium percentage or helium fraction in the breathing gas. |
| MOD | Maximum operating depth of a breathing gas is the depth at which the partial pressure of oxygen (pO2) of the gas mix exceeds a safe limit. |
| Multi level dive | A single or repetitive dive that includes time spent at various depths and therefore has no decompression limits that are not determined solely by the maximum depth reached. |
| Nitrox (Nx) | In sports diving, refers to any mix with a higher fraction of oxygen than standard air. |
| No deco | No decompression stop time. The maximum amount of time a diver may remain at a particular depth without having to make decompression stops during the subsequent ascent. |
| No decompression dive | Any dive which permits a direct, uninterrupted ascent to the surface at any time. |
| No dec time | Abbreviation for no decompression time limit. |
| OC | Open-circuit. Scuba that exhausts all exhaled gas. |
| OTU | Oxygen tolerance unit. Used to measure the whole-body-toxicity, caused by prolonged exposure to high oxygen partial pressures. The most common symptoms are irritation in the lungs, a burning sensation in the chest, coughing and reduction of the vital capacity. |
| O2% | Oxygen percentage or oxygen fraction in the breathing gas. Standard air has 21% oxygen. |
| pO2 | Partial pressure of oxygen. Limits the maximum depth to which the gas mixture can be safely used. The contingency partial pressure limit is 1.6 bar. Dives beyond this limit risk immediate oxygen toxicity. |
| Repetitive dive | Any dive whose decompression time limits are affected by residual nitrogen absorbed during previous dives. |
| Residual nitrogen | The amount of excess nitrogen remaining in a diver after one or more dives. |
| RGBM | Reduced gradient bubble model. Modern algorithm for tracking both dissolved and free gas in divers. |
| Scuba | Self-contained underwater breathing apparatus. |
| Surface time | Elapsed time between surfacing from a dive and beginning a descent for the subsequent dive. |
| Tissue group | Theoretical concept used to model bodily tissues for the construction of decompression tables or calculations. |
| Trimix | A breathing gas mix of helium, oxygen and nitrogen. |