water clock

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A clepsydra.

from The Century Dictionary.

• noun A clepsydra.

from the GNU version of the Collaborative International Dictionary of English.

• An instrument or machine serving to measure time by the fall, or flow, of a certain quantity of water; a clepsydra.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A device for measuring time by letting water flow out of a container, usually through a tiny aperture.
• noun A clepsydra.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun clock that measures time by the escape of water