rain barrel vs cistern

rain barrel and cistern both are nouns.

Word	Noun	Adjective	Verb	Adverb
rain barrel	Yes	No	No	No
cistern	Yes	No	No	No

As nouns, cistern is a hypernym of rain barrel; that is, cistern is a word with a broader meaning than rain barrel:

rain barrel: a barrel used as a cistern to hold rainwater
cistern: an artificial reservoir for storing liquids; especially an underground tank for storing rainwater

rain barrel (noun)	cistern (noun)
a barrel used as a cistern to hold rainwater	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