Source : Webster's Revised Unabridged Dictionary (1913)

Hardening \Hard"en*ing\, n.
   1. Making hard or harder.

   2. That which hardens, as a material used for converting the
      surface of iron into steel.

Harden \Hard"en\, v. t. [imp. & p. p. {Hardened}; p. pr. & vb.
   n. {Hardening}.] [OE. hardnen, hardenen.]
   1. To make hard or harder; to make firm or compact; to
      indurate; as, to harden clay or iron.

   2. To accustom by labor or suffering to endure with
      constancy; to strengthen; to stiffen; to inure; also, to
      confirm in wickedness or shame; to make unimpressionable.
      ``Harden not your heart.'' --Ps. xcv. 8.

            I would harden myself in sorrow.      --Job vi. 10.

Source : WordNet®

hardening
     n 1: abnormal hardening or thickening of tissue
     2: the process of becoming hard or solid by cooling or drying
        or crystallization; "the hardening of concrete"; "he
        tested the set of the glue" [syn: {solidifying}, {solidification},
         {set}, {curing}]
     3: the act of making something harder (firmer or tighter or
        more compact)