The Collaborative International Dictionary of English v.0.48:

Opisthodome \O*pis"tho*dome\, n. [L. opisthodomus, Gr. ?;
   'o`pisqen behind + do`mos house: cf. F. opisthodome.] (Arch.)
   A back chamber; especially, that part of the naos, or cella,
   farthest from the main entrance, sometimes having an entrance
   of its own, and often used as a treasury.
   [1913 Webster]