air sac

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An air-filled space in the body of a bird that forms a connection between the lungs and bone cavities and aids in breathing and temperature regulation.
2. n. See alveolus.
3. n. A saclike, thin-walled enlargement in the trachea of an insect.

Century Dictionary and Cyclopedia

1. n. In ornithology, a large air-cell; an air-space, an air-receptacle, or a pneumatocyst; one of the membranous bags or receptacles of air lodged in the hollow bones and the cavities of the body of birds, and communicating with the lungs.
2. n. plural The elongated cavities forming the ultimate branches of the air-passages in the lungs of mammals. Also called infundibula.
3. n. In botany, a cavity in a pollen-grain of the genus Pinus.

GNU Webster's 1913

1. (Anat.) One of the spaces in different parts of the bodies of birds, which are filled with air and connected with the air passages of the lungs; an air cell.
2. An alveolus.

WordNet 3.0

1. n. a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways
2. n. any of the thin-walled extensions of the tracheae of insects
3. n. any of the membranous air-filled extensions of the lungs of birds