Statistical measure of sea state: average height of the highest one-third of waves observed.
Regulatory detail & full definition
The standard wave-height value reported in all marine forecasts (NOAA, ECMWF, Met Office). A few individual waves can be ~1.7× Hs (10% probability) or even ~2× Hs (rogue waves, ~1 in 3,000 waves). Used by routing software and stability assessments.