Ceiling

The shallowest depth a diver in decompression may ascend to; going above it risks decompression sickness because tissues are still off-gassing.

In decompression terms, the ceiling is the shallowest depth you are allowed to reach at any moment of the ascent. Once a dive requires decompression, your tissues hold so much dissolved gas that ascending above a certain depth would let bubbles grow faster than your body can tolerate. The ceiling is that boundary. Dive computers display it explicitly when you enter deco, and it gradually rises toward the surface as you off-gas at your stops.

Respecting the ceiling is what a deco stop actually is: you hold just below the ceiling until the computer clears you to the next, shallower stop. Violating it, even briefly, increases the risk of decompression sickness, and many computers respond with alarms or by penalizing the rest of the schedule. Good buoyancy control matters here, since surge, an inattentive moment, or an uncontrolled DSMB launch can pull you above the line.

The word also has a literal meaning in overhead environments: in cave diving, cavern diving, and wreck penetration, the physical ceiling prevents a direct ascent entirely. Both meanings share the same consequence, namely that the surface is not immediately available, which is why both demand specific training. Training for those environments treats the ceiling as the defining hazard and builds every procedure around it.