A choir ( KWIRE; also known as a chorale or chorus) is a musical ensemble of singers. Choral music, in turn, is the music written specifically for such an ensemble to perform. Choirs may perform music from the classical music repertoire, which spans from the medieval era to the present, or popular music repertoire. Most choirs are led by a conductor, who leads the performances with arm, hand, and facial gestures.

The term choir is very often applied to groups affiliated with a church (whether or not they actually occupy the quire), whereas a chorus performs in theatres or concert halls, but this distinction is not rigid. Choirs may sing without instruments, or accompanied by a piano, pipe organ, a small ensemble, or an orchestra.

A choir can be a subset of an ensemble; thus one speaks of the "woodwind choir" of an orchestra, or different "choirs" of voices or instruments in a polychoral composition. In typical 18th century to 21st century oratorios and masses, 'chorus' or 'choir' implies that there is more than one singer per part, in contrast to the quartet of soloists also featured in these works.