The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]