1. The frequency of any time-harmonic (or sinusoidal) physical quantity governed by a wave equation. 2. In simple terms, the number of wave crests that pass a fixed point per second of time, equal to the inverse of the wave period. However, in a random sea, a variety of interpretations of this are possible. A commonly used definition is the so-called zero-upcrossing frequency, which is the number of successive upcrossings of the mean water level that pass a fixed point per second of time. See wave height, wave length.