What is "retrochoir"

Retrochoir \Re"tro*choir\, n. [Pref. 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.

n. 1 (context architecture English) Any extension of a church behind the high altar, such as a chapel. 2 (context architecture English) In an apsidal church, all the space beyond the line of the back or eastern face of the altar.