1. Any of various small insectivorous mammals of the family Talpidae, usually living underground and having thickset bodies with light brown to dark gray silky fur, rudimentary eyes, tough muzzles, and strong forefeet for burrowing.
2. A machine that bores through hard surfaces, used especially for tunneling through rock.
3. A spy who operates from within an organization, especially a double agent operating against his or her own government from within its intelligence establishment.

[Middle English molle, possibly akin to MOLD³ .]

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.