1. a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways (Freq. 5) • Syn: air sac, air cell • Derivationally related forms: alveolate • Hypernyms: sac • Part Holonyms: lung 2. a bony socket in the alveolar ridge that holds a tooth • Syn: tooth socket • Hypernyms: socket • Part Holonyms: alveolar ridge, gum ridge, alveolar process