What is the meaning of hardening?

noun

• Abnormal hardening or thickening of tissue.
• The process of becoming hard or solid by cooling or drying or crystallization.
  • usage: "the hardening of concrete"; "he tested the set of the glue"
  • synonyms: solidifying, solidification, set, curing
• The act of making something harder (firmer or tighter or more compact. )

verb

• Become hard or harder.
  • usage: "The wax hardened"
  • synonyms: harden, indurate
• Make hard or harder.
  • usage: "The cold hardened the butter"
  • synonyms: harden, indurate
• Harden by reheating and cooling in oil.
  • usage: "temper steel"
  • synonyms: temper, harden
• Make fit.
  • usage: "This trip will season even the hardiest traveller"
  • synonyms: season, harden
• Cause to accept or become hardened to; habituate.
  • usage: "He was inured to the cold"
  • synonyms: inure, harden, indurate