Alveolus

1. n. A small depression, sac, or vesicle, as the socket of a tooth, the air cells of the lungs, the ultimate saccules of glands, etc.
2. n. A cell in a honeycomb.
3. n. A small cavity in a coral, shell, or fossil

The word meanings were obtained from OPTED(The Online Plain Text English Dictionary), which is based on “The Project Gutenberg Etext of Webster’s Unabridged Dictionary” which is in turn based on the 1913 US Webster’s Unabridged Dictionary (See Project Gutenburg), as a text file.