The Collaborative International Dictionary of English v.0.48:

Antechoir \An"te*choir`\, n. (Arch.)
   (a) A space inclosed or reserved at the entrance to the
       choir, for the clergy and choristers.
   (b) Where a choir is divided, as in some Spanish churches,
       that division of it which is the farther from the
       sanctuary.
       [Webster 1913 Suppl.]