To harden means to become firm, solid, or more difficult to break, bend, or damage. When wet concrete hardens, it transforms from a thick liquid into solid stone. When you leave clay out in the sun, it hardens into a brittle material. Metal workers harden steel by heating it and then cooling it rapidly, making it stronger and more durable for tools or construction.

The word also describes becoming tougher emotionally or less affected by difficulty. A soldier might harden during training, learning to stay calm under pressure. Someone who faces repeated disappointments might harden their heart, becoming less trusting or less willing to show vulnerability. Parents sometimes worry when their once-gentle child starts to harden after being bullied, building walls to protect themselves.

You can harden your resolve, meaning you become more determined and less likely to give up. If you harden your position in an argument, you become more stubborn and less willing to compromise. Athletes harden their bodies through rigorous training, building strength and endurance.

The word suggests a permanent or semi-permanent change. Butter softens when warm but hardens again in the refrigerator. Attitudes that harden can take real effort to soften again.