Phase

In voltage and current, when there are multiple waveforms with matching periods, the numerical value that indicates the time shift between the starting points of each waveform is expressed in degrees or radians. In AC voltage, phase is important, and when operating generators in parallel, not only must the waveforms of each voltage match completely, but their phases must also align perfectly. This complete phase alignment is called being in phase. In three-phase AC voltage, the phases of each voltage waveform are positioned with a 120-degree (2/3π rad) difference.