The Collaborative International Dictionary of English v.0.48:
Water clock \Wa"ter clock`\ An instrument or machine serving to measure time by the fall, or flow, of a certain quantity of water; a clepsydra. [1913 Webster]WordNet (r) 3.0 (2006):
water clock n 1: clock that measures time by the escape of water [syn: water clock, clepsydra, water glass]