measure of the amount of energy associated with the fluctuation of the sea surface elevation per unit frequency band and per unit directional sector.
NOTE 1 The wave frequency spectrum (integrated over all directions) is often described by use of some parametric form such as the Pierson-Moskowitz or JONSWAP wave spectrum.
NOTE 2 The area under the wave spectrum is the zeroth spectral moment m0, which is a measure of the total energy in the sea state; m0 is used in contemporary definitions of the significant wave height
Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards