space in choir behind high altar
• (n.) Any extension of a church behind the high altar, as a chapel; also, in an apsidal church, all the space beyond the line of the back or eastern face of the altar.
Re'tro·choir noun [ Prefix retro- + choir .] (Eccl. Arch.) Any extension of a church behind the high altar, as a chapel; also, in an apsidal church, all the space beyond the line of the back or eastern face of the altar.