Definition of "alveolus"

A small cavity or pit.

1. (anatomy) Any of the many tiny air sacs in the lungs in which the bronchioles terminate and where oxygen and carbon dioxide are rapidly exchanged with the pulmonary capillaries.  examples

2. (anatomy, dentistry) The bony socket in which the root of a tooth resides.  examples

3. (anatomy) An acinus (saclike cavity) in a gland.  examples

4. (entomology) The socket of a macrotrichium (seta).