The Collaborative International Dictionary of English v.0.48:

Hypocaust \Hyp"o*caust\, n. [L. hypocaustum, Gr. ?; ? under + ?
   to burn: cf. F. hypocauste.] (Anc. Arch.)
   A furnace, esp. one connected with a series of small chambers
   and flues of tiles or other masonry through which the heat of
   a fire was distributed to rooms above. This contrivance,
   first used in bath, was afterwards adopted in private houses.
   [1913 Webster]