An instrument that keeps time exactly enough to be trusted. Its worth is entirely in not drifting.
Greek khronos (time) + metron (a measure); coined in the 18th century for the precision timekeepers built to solve the problem of longitude at sea.
Accuracy at a single instant is not enough — a stopped clock is exactly right twice a day and useless the rest of it, so what a chronometer promises is correctness sustained across every instant, which is a far harder thing to keep than correctness at any one of them.