A phase refers to position information of a wave that fluctuates periodically. For instance, in a cosine wave (or a sine wave) such as input signal Acos(ωt+φ), the phase refers to (ωt+φ), which is the angle (phase angle). Here, phase φ of t=0 is referred to as an initial phase and normally, f is referred to as a phase.

If φ is positive based on the cosine waveform as the reference, the display will be phase advance and if it is negative, the display will be phase delay. The representation of the phase component as a frequency function is referred to as a phase spectrum and means the phase angle under 1ch analysis performed using the data import point as the reference. In the circular measure, since the unit of a phase angle is rad (radian), the vertical axis of the phase spectrum is -π < φ < π, however, in the FFT analyzer, normally it is expressed in the degree measure, which is -180 deg < φ < 180 deg.