water clock vs clepsydra

water clock and clepsydra both are nouns.

Word	Noun	Adjective	Verb	Adverb
water clock	Yes	No	No	No
clepsydra	Yes	No	No	No

As nouns, water clock and clepsydra are synonyms defined as:

water clock and clepsydra: clock that measures time by the escape of water

Other synonyms of water clock include water glass.

water clock (noun)	clepsydra (noun)
clock that measures time by the escape of water	clock that measures time by the escape of water