cistern vs rain barrel

cistern and rain barrel both are nouns.

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

As nouns, rain barrel is a hyponym of cistern; that is, rain barrel is a word with a more specific, narrower meaning than cistern:

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

Other hyponyms of cistern include cesspit, cesspool, sink, sump.

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