1. A small inflamed elevation of the skin that is filled with pus; a pimple.
2. A small swelling similar to a blister or pimple.
3. Something likened to an inflamed, pus-filled lesion: "a cool glimpse of green between hot pustules of sooty sprawl" (Nicholas Proffitt).

[Middle English, from Old French, from Latin pustula, blister.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.