in physics, it is the state-evolving periodic quality or process that may vary in a sinusoidal fashion such that after a minimum time period, the order parameter assumes the same value. When periodic quantities have the same waveform (or frequency), they are in-phase, otherwise they are out-of-phase, and which includes the more specific circumstances of anti-phase. The difference between two sinusoidal quantities gives a measure of relative phase that can be expressed as a phase angle difference.