Group Delay
A measurement of the amount of phase delay induced by a filter at various frequencies. Ideally, a filter would pass or attenuate all signals without any changes in the phase of the input signal. In the real world this is not the case, and differing amounts of delay will be induced at varying frequencies. By examination of a group delay plot we can see how phase has been affected, and we can detect problems that may cause phase distortion or "smearing" of the signal.