Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

clepsydra
     n : clock that measures time by the escape of water [syn: {water
         clock}, {water glass}]
     [also: {clepsydrae} (pl)]