water tank vs cistern

water tank and cistern both are nouns.

Word	Noun	Adjective	Verb	Adverb
water tank	Yes	No	No	No
cistern	Yes	No	No	No

As nouns, water tank and cistern are synonyms defined as:

water tank and cistern: a tank that holds the water used to flush a toilet

water tank (noun)	cistern (noun)
a tank that holds the water used to flush a toilet	an artificial reservoir for storing liquids; especially an underground tank for storing rainwater
	a tank that holds the water used to flush a toilet
	a sac or cavity containing fluid especially lymph or cerebrospinal fluid