Adjacent-channel interference: Extraneous power from a signal in an adjacent channel.

Note 1: Adjacent channel interference may be caused by inadequate filtering, such as incomplete filtering of unwanted modulation products in frequency modulation (FM) systems, improper tuning, or poor frequency control, in either the reference channel or the interfering channel, or both.

Note 2: Adjacent-channel interference is distinguished from crosstalk.

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