WordWeb Online

1. A tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways
   - alveolus, air cell
    
2. Any of the thin-walled extensions of the tracheae of insects
    
3. Any of the membranous air-filled extensions of the lungs of birds

Derived forms: air sacs

Type of: sac

Part of: bird, birdie [informal], insect, lung

Encyclopedia: Air sac