alveolus

noun5

1. •
   A small cavity or pit.
2. •
   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.
3. •
   The bony socket in which the root of a tooth resides.
4. •
   An acinus (saclike cavity) in a gland.

   • The sprouts proliferate and canalize to form the lactiferous ducts and the outbuddings form the alveoli of the mammary glands. — 1997
5. •
   The socket of a macrotrichium (seta).