Watches : What is a unidirectional bezel?

Often the bezel (top ring on the case), serves to record additional data, and can often move in both directions to provide a number of functions. A unidirectional bezel only turns one way to prevent any danger of false manoeuvre. Especially important when being used to measure diving times as even if the bezel is knocked and moved it will simply indicate the diver has less air or decompression time rather than more.