1570s, "note having the same pitch as another; identity in pitch of two or more sounds; interval between tones of the same pitch," especially the interval of an octave, from Middle French unisson "unison, accord of sound" (16c.) or directly from Medieval Latin unisonus "having one sound, sounding the same," from Late Latin unisonius "in immediate sequence in the scale, monotonous," from Latin uni- "one" (from PIE root *oi-no- "one, unique") + sonus "sound" (from PIE root *swen- "to sound"). Figurative sense of "harmonious agreement" is first attested 1640s.
