(electrical) A group of alternating current circuits having two or more interrelated voltages, usually of equal amplitudes, phase differences, and periods, etc; if a neutral conductor exists, the voltages referenced to the neutral are equal in amplitude and phase; the most common version is that of 3-phase, equal in amplitude with phases 120 degrees apart.

A polyphase system (the term coined by Silvanus Thompson) is a means of distributing alternating-current electrical power that utilizes more than one AC phase, which refers to the phase

A polyphase quadrature filter, or PQF, is a filter bank which splits an input signal into a given number N (mostly a power of 2) of equidistant sub-bands.