Noun, pl. alveoli
An anatomical structure that has the form of a hollow cavity. (wikipedia.org)
A small air sac in the lungs, where oxygen and carbon dioxide are exchanged with the blood. (wiktionary.org)
2. A small cell containing air in the lungs, a sac-like dilation of the alveolar ducts in the lung. (biology-online.org)
3. Any of the many tiny air sacs in the lungs where the exchange of oxygen and carbon dioxide takes place. (Google Dictionary)
Word origin: From Latin alveolus, a diminutive of alveus (“a tray, trough, basin”), from alvus (“the belly, the stomach, bowels, womb, etc.”).