sac vs cyst

sac and cyst both are nouns.

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

As nouns, cyst is a hyponym of sac; that is, cyst is a word with a more specific, narrower meaning than sac:

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

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