/yooh"neuh seuhn, -zeuhn/ , n.
1. coincidence in pitch of two or more musical tones, voices, etc.
2. the musical interval of a perfect prime.
3. the performance of musical parts at the same pitch or at the octave.
4. a sounding together in octaves, esp. of male and female voices or of higher and lower instruments of the same class.
5. a process in which all elements behave in the same way at the same time; simultaneous or synchronous parallel action: to march in unison.
6. in unison , in perfect accord; corresponding exactly: My feelings on the subject are in unison with yours.
[ 1565-75; unisonus of a single sound, equiv. to L uni- UNI- + sonus sound ]