Meaning of alveolus

al•ve•o•lus

1. a little cavity, pit, or cell, as a cell of a honeycomb.
2. an air cell of the lungs, formed by the terminal dilation of tiny air passageways.
3. one of the terminal secretory units of a racemose gland.
4. the socket within the jawbone in which the root or roots of a tooth are set.