air sac

WordReference Random House Unabridged Dictionary of American English © 2026

air′ sac′, 

1. a sac containing air.
2. Anatomy alveolus (def. 2).
3. any of certain cavities in a bird's body connected with the lungs.
4. a saclike dilation of the wall of a trachea in many insects.
• 1820–30

Collins Concise English Dictionary © HarperCollins Publishers::

air sac noun

1. any of the membranous air-filled extensions of the lungs of birds, which increase the efficiency of gaseous exchange in the lungs