sac vs air sac

sac and air sac both are nouns.

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

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

sac (noun)	air sac (noun)
a structure resembling a bag in an animal	any of the membranous air-filled extensions of the lungs of birds
a case or sheath especially a pollen sac or moss capsule	any of the thin-walled extensions of the tracheae of insects
an enclosed space	a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways