In most electrical circuits, desired signals are transferred by a differential voltage between two conductors.
When referenced to the local common or ground, a common-mode signal appears on both lines of a two-wire cable, in phase and with equal amplitudes.
Likewise, RF noise transmitted from a cable tends to emanate from both conductors.
Elimination of common-mode signals on cables entering or leaving electronic equipment is important to ensure electromagnetic compatibility.
This article incorporates public domain material from Federal Standard 1037C.