1574, from M.Fr. unisson "unison, accord of sound" (16c.), from M.L. unisonus "having one sound, sounding the same," from L.L. unisonius "in immediate sequence in the scale, monotonous," from L. uni- "one" (see one) + sonus "sound" (see sound (n.1)).
Sense of "harmonious agreement" is first attested 1650.
Playing or singing the same musical notes, or notes separated from each other by one or several octaves. Musicians who perform in unison are not playing or singing chords.