An Alveolus

1. A small cavity or pit.
2. (anatomy) An anatomical structure that has the form of a hollow cavity, as:

• (anatomy, pulmonology) A small air sac in the lungs, where oxygen and carbon dioxide are exchanged with the blood.

Synonym: pulmonary alveolus

• (anatomy, dentistry) A dental alveolus (tooth socket): the socket in which a tooth resides.
• (entomology) The socket of a macrotrichium (seta).