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: 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.

1. The act or process of becoming hard or harder.
2. Something that hardens, as a substance added to iron to yield steel.
3. Gradual exposure of plants to cold weather.