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)

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "hardening":
   abidingness, acclimation, acclimatization, accommodation,
   accustoming, adaption, adjustment, agglutination, arteriosclerosis,
   atherosclerosis, breaking, breaking-in, calcification, callusing,
   case hardening, changelessness, clumping, clustering, compression,
   concentration, concretion, condensation, conditioning,
   consolidation, constancy, cornification, crystallization,
   densification, domestication, durability, durableness, duration,
   endurance, familiarization, firming, firmness, fixedness,
   fortification, fossilization, frozenness, granulation, habituation,
   hornification, housebreaking, immobility, immovability,
   immovableness, immutability, induration, indurative, inurement,
   invariability, invariableness, inveteracy, invigoration,
   lapidification, lastingness, lithification, long standing,
   naturalization, orientation, ossification, permanence, permanency,
   perpetualness, persistence, persistency, petrifaction,
   petrifactive, petrification, petrifying, quiescence, refreshment,
   reinforcement, reinvigoration, restrengthening, revivification,
   rigidity, sclerosis, seasoning, setting, solidification, solidity,
   stability, standing, stasis, steadfastness, steeling, stiffening,
   strengthening, taming, tempering, torpor, toughening, training,
   unchangeability, unchangingness, vitrifaction, vitrification