cistern vs sac

cistern and sac both are nouns.

Word	Noun	Adjective	Verb	Adverb
cistern	Yes	No	No	No
sac	Yes	No	No	No

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

cistern: a sac or cavity containing fluid especially lymph or cerebrospinal fluid
sac: a structure resembling a bag in an animal

cistern (noun)	sac (noun)
an artificial reservoir for storing liquids; especially an underground tank for storing rainwater	a structure resembling a bag in an animal
a tank that holds the water used to flush a toilet	a case or sheath especially a pollen sac or moss capsule
a sac or cavity containing fluid especially lymph or cerebrospinal fluid	an enclosed space