air sac

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun A saclike, thin-walled enlargement in the trachea of an insect.
• noun An inflatable pouch attached to the larynx in certain mammals, including the nonhuman great apes.

from The Century Dictionary.

• noun In botany, a cavity in a pollen-grain of the genus Pinus.
• noun 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.
• noun plural The elongated cavities forming the ultimate branches of the air-passages in the lungs of mammals. Also called infundibula.

from the GNU version of the Collaborative International Dictionary of English.

• (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.
• An alveolus.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways
• noun any of the thin-walled extensions of the tracheae of insects
• noun any of the membranous air-filled extensions of the lungs of birds