air cell

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(electronics) A cell in which depolarization at the positive electrode is accomplished chemically by reduction of the oxygen in the air.
(mechanical engineering) A small auxiliary combustion chamber used to promote turbulence and improve combustion in certain types of diesel engines.
(zoology) A cavity or receptacle for air such as an alveolus, an air sac in birds, or a dilation of the trachea in insects.

1. A terminal dilation of the bronchiole where gas exchange is thought to occur.
2. Spaces that contain air in the bones of the skull.

The noun has one meaning:

Meaning #1: a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways
  Synonyms: alveolus, air sac