alveolus alveolus /ælˈviːələs/

noun

1. () A tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways  ( air cell , air sac , alveolus )
   a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways
     The alveolus is the tiny air sac in the lungs where oxygen and carbon dioxide exchange takes place.
   The alveolus is the tiny air sac in the lungs where oxygen and carbon dioxide exchange takes place.
   
2. A bony socket in the alveolar ridge that holds a tooth  ( alveolus , tooth socket )
   a bony socket in the alveolar ridge that holds a tooth