air sac vs alveolus

air sac and alveolus both are nouns.

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

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

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

Other synonyms of air sac include air cell.

air sac (noun)	alveolus (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 bony socket in the alveolar ridge that holds a tooth
a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways