The Collaborative International Dictionary of English v.0.48:

Alveolus \Al*ve"o*lus\ ([a^]l*v[=e]"[-o]*l[u^]s), n.; pl.
   Alveoli (-l[imac]). [L., a small hollow or cavity, dim. of
   alveus: cf. F. alv['e]ole. See Alveary.]
   1. A cell in a honeycomb.
      [1913 Webster]

   2. (Zool.) A small cavity in a coral, shell, or fossil.
      [1913 Webster]

   3. (Anat.) A small depression, sac, or vesicle, as the socket
      of a tooth, the air cells of the lungs, the ultimate
      saccules of glands, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

alveolus
    n 1: a tiny sac for holding air in the lungs; formed by the
         terminal dilation of tiny air passageways [syn: alveolus,
         air sac, air cell]
    2: a bony socket in the alveolar ridge that holds a tooth [syn:
       tooth socket, alveolus]