The following was originally from the 1911 Encyclopedia Britannica. As such, it may not yet be properly adapted to Wikipedia (obsolete, typos, no accidental links, etc.). It is included as a stub of sorts to inspire people to modify it:
BACK-CHOIR, RETRO-CHOIR, a space behind the high altar in the choir of a church, in which there is, or was, a small altar standing back to back with the other.