Crest and trough

The crest is the point on a wave with the maximum value of upward displacement within a wave-cycle. A trough is the opposite of a crest, so the minimum or lowest point in a wave-cycle.

  • On a surface wave, the crest is a point where the displacement of the medium (water for example) is at a maximum. A trough is the opposite of a crest, so a trough is a point where the displacement of the medium is at a minimum.
  • In the case of an electromagnetic wave which has an electric and a magnetic axis then a crest on either axis refers to maximum displacement in the positive direction whilst a trough refers to minimum displacement.

https://en.wikipedia.org/wiki/Crest_and_trough