Phase distortion

Phase distortion: Distortion that occurs when (a) the phase-frequency characteristic[?] is not linear over the frequency range[?] of interest, i.e. , the phase shift introduced by a circuit or device is not directly proportional to frequency, or (b) the zero-frequency intercept of the phase-frequency characteristic is not 0 or an integral multiple of 2π radians. Synonym phase-frequency distortion.

Source: from Federal Standard 1037C and from MIL-STD-188