1. A type of electronic modulation in which the phase of a carrier wave is varied in order to transmit the information contained in the signal.
2. Varying the angle of a wave in a carrier in order to transmit analog or digital data.

For digital signals, phase modulation (PM) is widely used in conjunction with amplitude modulation (AM); for example, quadrature amplitude modulation (QAM) uses both phase and amplitude modulation to create different binary states for transmission.

The modulation (variation of amplitude or frequency) of the phase of a light wave; such as, by changing the index of refraction and the velocity of propagation and hence the phase at a point in the medium in which the wave is propagating, in accordance with an applied field serving as the modulating signal.