The Collaborative International Dictionary of English v.0.48:

Harden \Hard"en\ (h[aum]rd"'n), v. t. [imp. & p. p. Hardened
   (-'nd); p. pr. & vb. n. Hardening (-'n*[i^]ng).] [OE.
   hardnen, hardenen.]
   1. To make hard or harder; to make firm or compact; to
      indurate; as, to harden clay or iron.
      [1913 Webster]

   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.
      [1913 Webster]

            I would harden myself in sorrow.      --Job vi. 10.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hardening \Hard"en*ing\, n.
   1. Making hard or harder.
      [1913 Webster]

   2. That which hardens, as a material used for converting the
      surface of iron into steel.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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: hardening, solidifying,
       solidification, set, curing]
    3: the act of making something harder (firmer or tighter or more
       compact)