noun

• (architecture) Uncommon form of quire (“one quarter of a cruciform church, or the architectural area of a church used by the choir, often near the apse”).
  "Here, in the transept and choir, where the service was being held, one was conscious every moment of an increasing brightness; colours glowing vividly beneath the circular chandeliers, and the rows of small lights on the choristers' desks flashed and sparkled in front of the boys' faces, deep linen collars, and red neckbands."
• (Christian angelology) One of the nine ranks or orders of angels.
  "Seraphim, Cherubim, and Thrones are three of the choirs of angels."

verb

• (intransitive) To sing in concert.
  "The great aim of this book is to secure congregational singing, which the churches must come to, at last, after a long interval of choiring."