The distribution of wave energy (for ocean surface waves) with frequency (1/ period) and direction. The 2D spectrum can be expressed either as a function of frequency and direction or as a function of the 2D wave vector. Integrated over direction, the 1D spectrum can be expressed as a function of frequency or of wavenumber. For gravity waves the square of the wave height is proportional to the potential energy of the sea surface and the energy spectrum is equivalent to the spectrum of sea surface fluctuations. See spectrum.