The Collaborative International Dictionary of English v.0.48:

Reredos \Rere"dos\ (r?r"d?s), n. [From rear + F. dos back, L.
   dorsum. Cf. Dorsal.] (Arch.)
   (a) A screen or partition wall behind an altar.
   (b) The back of a fireplace.
   (c) The open hearth, upon which fires were lighted,
       immediately under the louver, in the center of ancient
       halls. [Also spelt reredosse.] --Fairholt.
       [1913 Webster]

WordNet (r) 3.0 (2006):

reredos
    n 1: a painted or carved screen placed above and behind an altar
         or communion table [syn: altarpiece, reredos]