Alveolus: One of the tiny air sacs located at the very ends of the bronchioles within the lungs where the exchange of oxygen and carbon dioxide takes place. The alveolus looks like a cell in a honeycomb.

The word comes from the Latin diminutive of "alveus" meaning a cavity or hollow = a little cavity or hollow. The plural of alveolus is alveoli and the adjective is alveolar.