Noun

1. Any utterance produced simultaneously by a group
2. A group of people assembled to sing together
3. The part of a song where a soloist is joined by a group of singers
4. A body of dancers or singers who perform together
5. A company of actors who comment (by speaking or singing in unison) on the action in a classical greek play

Verb

1. Utter in unison
2. Sing in a choir