1. (symbol: ϕ) The difference in phase between two sinusoidally varying quantities of the same frequency. It may be expressed as an angle (the phase angle) or as a time.

2. The angle between the reversed secondary vector of an instrument transformer and the corresponding primary vector. The vectors represent current in a current transformer and voltage in a voltage transformer. The phase difference is positive if the reversed secondary vector leads the primary and negative if it lags. The term phase error has been used in this application but this is deprecated.