al·ve·o·lus 
(ăl-vēə-ləs)
(ăl-vēə-ləs)n. pl. al·ve·o·li (-lī′)
1. A small angular cavity or pit, such as a honeycomb cell.
2. A tooth socket in the jawbone.
3. A tiny, thin-walled, capillary-rich sac in the lungs where the exchange of oxygen and carbon dioxide takes place. Also called air sac.
[Latin, small hollow, diminutive of alveus, a hollow, from alvus, belly.]