1. The period of any time-harmonic (or sinusoidal) physical quantity governed by a wave equation. 2. The time interval between successive wave crests or troughs. However, in a random sea, a variety of interpretations of this are possible. A commonly used definition is the so-called zero-upcrossing period, which is the time interval between two successive upcrossings of the mean water level. See wave frequency, wave height, wave length.