air cell

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

• (Bot.) A cavity in the cellular tissue of plants, containing air only.
• (Anat.) A receptacle of air in various parts of the system; as, a cell or minute cavity in the walls of the air tubes of the lungs; the air sac of birds; a dilatation of the air vessels in insects.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology an air pocket at the bottom end of an egg formed from contraption of the egg's contents through loss of moisture and cooling from its initial filled state.

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