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]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "alveolus":
   alveolar ridge, alveolation, antrum, apex, armpit,
   arytenoid cartilages, back, basin, blade, bowl, cavity, concave,
   concavity, crater, crypt, cup, dent, depression, dimple, dint, dip,
   dorsum, fold, follicle, funnel chest, furrow, gouge, hard palate,
   hole, hollow, hollow shell, honeycomb, impress, impression,
   imprint, indent, indentation, indention, indenture, lacuna, larynx,
   lips, nasal cavity, notch, oral cavity, palate, pharyngeal cavity,
   pharynx, pit, pock, pocket, pockmark, print, punch bowl, scoop,
   shell, sink, sinus, socket, soft palate, speech organ, sunken part,
   syrinx, teeth, teeth ridge, tip, tongue, trough, velum,
   vocal chink, vocal cords, vocal folds, vocal processes, voice box,
   vug