u·ni·son / ˈyoōnəsən; -zən/ • n. 1. simultaneous performance of action or utterance of speech: “Yes, sir,” said the girls in unison. 2. Mus. coincidence in pitch of sounds or notes: the flutes play in unison with the violas. ∎  a combination of notes, voices, or instruments at the same pitch or (esp. when singing) in octaves: good unisons are formed by flutes, oboes, and clarinets.• adj. performed in unison.DERIVATIVES: u·nis·o·nous / yoōˈnisənəs/ adj.

unison (mus.) identity of pitch XVI; exact agreement XVII. — (O)F. unison (mod. unisson) or late L. ūnisonus of the same sound, f. UNI- + sonus SOUND3.

unison. Sounding of the same note by all perf., e.g. unison singing, everyone singing the same tune but not in harmony.