cyst vs sac

cyst and sac both are nouns.

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

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

cyst: a small anatomically normal sac or bladderlike structure (especially one containing fluid)
sac: a structure resembling a bag in an animal

cyst (noun)	sac (noun)
a small anatomically normal sac or bladderlike structure (especially one containing fluid)	a structure resembling a bag in an animal
a closed sac that develops abnormally in some body structure	a case or sheath especially a pollen sac or moss capsule
	an enclosed space