Phase jitter is a rapid, repeated phase perturbation resulting in the intermittent shortening or lengthening of an electronic signal. Phase jitter may be random or cyclic.

The phase departure in phase jitter usually is smaller, but more rapid, than that of phase perturbation. Phase jitter may be expressed in degrees, radians, or seconds. Phase jitter is usually random. However, if cyclic, phase jitter may be expressed in hertz as well as in degrees, radians, or seconds.

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