cisterna vs sac

cisterna and sac both are nouns.

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

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

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

cisterna (noun)	sac (noun)
a sac or cavity containing fluid especially lymph or cerebrospinal fluid	a structure resembling a bag in an animal
	a case or sheath especially a pollen sac or moss capsule
	an enclosed space