1. (

• alveolus ,
• air_sac ,
• air_cell

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

2. (

• air_sac ,

) any of the thin-walled extensions of the tracheae of insects ;

3. (

• air_sac ,

) any of the membranous air-filled extensions of the lungs of birds ;