1. Undesired coupling of a signal from one circuit, part of a circuit, or channel, to another. 2. Any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Note 1: In telephony, crosstalk is usually distinguishable as speech or signaling tones. Note 2: In video, "ghost" images from one source appear in addition to the signals of interest transmitted from another.