Unidirectional bezel

Often the bezel (the ring fitted around the case) is used to record additional information such as the duration of a given event. It can be rotated in both directions.

A unidirectional bezel on the other hand rotates in one direction only. It was designed as a safety feature. For example, if a diver wants to measure a dive time and the bezel is accidentally knocked while underwater, it will show less rather than more remaining air or decompression time.