harden

verb

hard·​en ˈhär-dᵊn 

hardened; hardening ˈhärd-niŋ 

ˈhär-dᵊn-iŋ

transitive verb

1

: to make hard or harder

2

: to confirm in disposition, feelings, or action

especially : to make callous

hardened his heart

3

a

: inure, toughen

harden troops

b

: to inure to unfavorable environmental conditions (such as cold)

… work to mitigate future fires by clearing trees and hardening homes.

—Jake Bittle

—often used with off

harden off seedlings before transplanting

4

: to protect from blast, heat, or radiation (as by a thick barrier or placement underground)

intransitive verb

1

: to become hard or harder

2

a

: to become firm, stable, or settled

b

: to assume an appearance of harshness or severity

Her face hardened at the thought.

3

: to become gradually acclimatized to unfavorable conditions

—often used with off

plants hardened off before the first frost

Synonyms of harden

• freeze
• stiffen

Examples of harden in a Sentence

The presence of certain substances in the blood can cause the arteries to harden.

These additives are designed to harden the steel.

substances that can harden the arteries

The news has hardened opposition to the government.

He had been hardened by his years of military service.

Recent Examples on the Web

Inside Daymond John’s biohacking routine The clarity John gained from his battle with cancer and experimenting with weight loss in the past has hardened into a strict weekly routine. —Sydney Lake, Fortune, 8 Mar. 2026

The Defense Department has been exploring ways to harden positions and counter drones ever since the beginning of the Russia-Ukraine conflict four years ago. —James Laporta march 5, CBS News, 6 Mar. 2026

The lunar sea Mare Crisium (the Sea of Crisis) is just visible as a dark circular feature at the top of the sunlit lunar disk, where lava flooded a network of impact craters over a billion years ago, before hardening into a vast basaltic plain. —Anthony Wood, Space.com, 4 Mar. 2026

And on those higher ozone days, there were more hemorrhage (bleeding, as opposed to clotting) strokes and more plaque buildup causing major arteries to harden. —Theresa Gaffney, STAT, 4 Mar. 2026

Word History

First Known Use

13th century, in the meaning defined at transitive sense 1

Phrases Containing harden

• case-harden
• harden one's heart

Rhymes for harden

• cardan
• garden
• pardon

Kids Definition

harden

verb

hard·​en ˈhärd-ᵊn 

hardened; hardening ˈhärd-niŋ 

-ᵊn-iŋ

1

: to make or become hard or harder

2

: to make unfeeling or unsympathetic

hardened his heart

3

: to make or become hardy or strong

muscles hardened by exercise

4

: to protect from blast, heat, or radiation (as by a thick barrier or by placing underground)

hardened missile sites

5

a

: to become firm, stable, or settled

b

: to express harshness

hardener

ˈhärd-nər, 

-ᵊn-ər

noun

Biographical Definition

Harden 1 of 2

biographical name (1)

Har·​den ˈhär-dᵊn 

Sir Arthur 1865–1940 English chemist

Harden

2 of 2

biographical name (2)

Maximilian 1861–1927 originally Felix Ernst Witkowski German writer