1. To make hard or harder.
2. To enable to withstand physical or mental hardship.
3. To make unfeeling, unsympathetic, or callous: "To love love and not its meaning hardens the heart in monstrous ways" (Archibald MacLeish).
4. To make sharp, as in outline.
5. To protect (nuclear weapons) by surrounding with earth or concrete.

1. To become hard or harder.
2. To rise and become stable. Used of prices.
3. To become inured.

SYNONYMS   harden, acclimate, acclimatize, season, toughen. These verbs mean to make resistant to hardship, especially through continued exposure: was hardened to frontier life; is acclimated to the tropical heat; was acclimatized by long hours to overwork; became seasoned to life in prison; toughened by experience.

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.