Choir
A vocal ensemble with multiple singers per part; also a term for a piece written for such an ensemble.
What it means
A choir is a vocal ensemble in which each voice part is sung by several people, as opposed to a solo voice or a vocal ensemble in which each part has only one singer. The usual division is soprano, alto, tenor, and bass (SATB), though all-male, all-female, and children's choirs exist, as do choirs with more voice groups.
The word also refers to a choral section of a larger work, such as a “chorus” in a cantata or opera, as distinct from a recitative or aria.
Musical use
The term traces back to Greek theater, where the chorus sang and danced commentary on the stage action. In Western church music the choir became the main vehicle of the liturgy, from Gregorian chant through polyphonic motets and masses to the cantata. From the 19th century onward a broad amateur choral culture also grew up, with community, concert, and children's choirs.
A choir should be distinguished from a vocal ensemble, where each part is sung solo, and from a speaking chorus, which uses no fixed pitches at all. In opera, “chorus” refers both to the group of singers on stage and to the music written for them.
Performance practice
If you sing in a choir, focus on blending with your section: intonation, vowel color, and breath should match your neighbors' so the group sounds like one voice rather than several soloists stacked on top of each other.