General condition of the surface of the open sea. This is comprised of two core measurements, the wave height which is dependent on the local surface wind strength and thus termed the ‘wind sea’ and the swell which is a slow and regular movement of the sea in rolling waves that do not break and are in the process of dissipating as they travel from their relatively distant origin.

Source : https://www.thinkdefence.co.uk/2014/10/submarine-rescue/sea-state/