air cell vs air sac

air cell and air sac both are nouns.

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

As nouns, air cell and air sac are synonyms defined as:

air cell and air sac: a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways

Other synonyms of air cell include alveolus.

air cell (noun)	air sac (noun)
a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways	any of the membranous air-filled extensions of the lungs of birds
	any of the thin-walled extensions of the tracheae of insects
	a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways