The Collaborative International Dictionary of English v.0.48:

Clepsydra \Clep"sy*dra\ (?; 277), n. [L. from Gr. ?; ? to steal,
   conceal + ? water.]
   A water clock; a contrivance for measuring time by the
   graduated flow of a liquid, as of water, through a small
   aperture. See Illust. in Appendix.
   [1913 Webster]

WordNet (r) 3.0 (2006):

clepsydra
    n 1: clock that measures time by the escape of water [syn:
         water clock, clepsydra, water glass]