Definitions of harden:

• verb:   become hard or harder

  Example: "The wax hardened"

• verb:   make hard or harder

  Example: "The cold hardened the butter"

• verb:   cause to accept or become hardened to; habituate
• verb:   make fit
• verb:   harden by reheating and cooling in oil
• name:  A surname (common: 1 in 9090 families; popularity rank in the U.S.: #1123)