air sac vs air cell

air sac and air cell both are nouns.

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

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

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

Other synonyms of air sac include alveolus.

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