What is a wave crest?

Question

What is a wave crest?

Answer 1

Crest and Trough

The section of the wave that rises above the undisturbed position is called the crest. That section which lies below the undisturbed position is called the trough.

Answer 2

if you are asking about a water wave crest then it is the high point of the wave.  The low point is the trough.  These are measured in relation to the still water level.  the difference between the crest and the trough is the wave height