Refers to the shift, expressed as the angle φ, between two sinusoidal variables or signals with the same angular frequency ω such as the input and for a transfer element. The phase shift is described by the initial or zero phase angle φ 0 which corresponds to the phase delay.
φ = ωt + φ 0
In this context, φ O > O phase advance (lead) and φ O < O phase delay (lag).