air sac

English definition

noun

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

   Source: WordNet 3.0

2. any of the thin-walled extensions of the tracheae of insects

   Source: WordNet 3.0

3. any of the membranous air-filled extensions of the lungs of birds

   Source: WordNet 3.0