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.
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   2. (Zool.) A small cavity in a coral, shell, or fossil.
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   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.
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