Definitions for "SIGNIFICANT WAVE HEIGHT"
Keywords:  crest, trough, height, highest, wave
Average height of the highest one-third of the WAVES for a stated interval of time.
Defined traditionally as the mean height of the highest third of the waves, but now usually defined as four times the root-mean-square of the surface elevation (or equivalently as four times the square root of the first moment of the wave spectrum).
The average height of the highest 33% of all waves in a swell train or in a wave generating region. It also approximates the value an experienced observer would report if visually estimating sea height. This is the predicted height given in the Coastal Waters Forecast.