Retrochoir

• (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.
Found on http://thinkexist.com/dictionary/meaning/retrochoir/

Retrochoir

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.
Found on http://www.encyclo.co.uk/webster/R/69

retrochoir

space in choir behind high altar
Found on http://phrontistery.info/r.html
No exact match found