Formal:
* Chorus: This is the most common and direct synonym.
* Cantata: This specifically refers to a choral composition.
* Choral group: This emphasizes the group aspect of the choir.
* Vocal ensemble: This is a broader term that includes any group of singers.
Informal:
* Singing group: This is a simple and straightforward term.
* Glee club: This term is often used for school choirs.
* Singing society: This emphasizes the community aspect of the choir.
The best synonym to use will depend on the specific context.