Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. (Zo["o]l.) A small cavity in a coral, shell, or fossil

   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.

Source : WordNet®

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