How long you can stay at a given depth before mandatory decompression stops become unavoidable. Your dive computer recalculates it continuously.
Under pressure, nitrogen dissolves into your blood and tissue. The no-decompression limit is the boundary up to which you can shed it simply by ascending slowly, with no mandatory stops on the way. Two misunderstandings cost the most here. First, the limit is not a safety guarantee, it is the edge of a calculation model — decompression sickness happens inside the limit too, helped along by fast ascents, cold, exertion and not drinking enough. Second, not every computer calculates the same way, and your dives from the past few days count towards it, so two units side by side can show different times. When they disagree, go by the more conservative number. In practice: start your ascent in good time rather than when the display hits zero, and do not come up faster than ten metres per minute.