1. For any type of periodic motion (e.g., rotation, oscillation) a point or stage in the period to which the motion has advanced with respect to a given initial point. Specifically, the phase or phase angle is the angular measure along a simple harmonic wave, the linear distance of one wavelength being 360° of phase measure. This is often generalized by equating one cycle of any oscillation to 360°. See delay, interference, surface of constant phase. 2. The state of aggregation of a substance, for example, solid, liquid, or gas.