What is "significant wave height"

n. (context oceanographics nautical English) A standardized measure of trough-to-crest wave height calculated for the needs of seafarer; exact mathematical definition has varied over time and currently it is defined as four times the standard deviation of the surface elevation.

In physical oceanography, the significant wave height (SWH or H) is defined traditionally as the mean wave height ( trough to crest) of the highest third of the waves (H). Nowadays it is usually defined as four times the standard deviation of the surface elevation – or equivalently as four times the square root of the zeroth-order moment ( area) of the wave spectrum. The symbol H is usually used for that latter definition. The significant wave height may thus refer to H or H; the difference in magnitude between the two definitions is only a few percent.