The Collaborative International Dictionary of English v.0.48:

Alteration \Al`ter*a"tion\, n. [Cf. F. alt['e]ration.]
   1. The act of altering or making different.
      [1913 Webster]

            Alteration, though it be from worse to better, hath
            in it incoveniences.                  --Hooker.
      [1913 Webster]

   2. The state of being altered; a change made in the form or
      nature of a thing; changed condition.
      [1913 Webster]

            Ere long might perceive
            Strange alteration in me.             --Milton.
      [1913 Webster]

            Appius Claudius admitted to the senate the sons of
            those who had been slaves; by which, and succeeding
            alterations, that council degenerated into a most
            corrupt.                              --Swift.
      [1913 Webster]

WordNet (r) 3.0 (2006):

alteration
    n 1: an event that occurs when something passes from one state
         or phase to another; "the change was intended to increase
         sales"; "this storm is certainly a change for the worse";
         "the neighborhood had undergone few modifications since his
         last visit years ago" [syn: change, alteration,
         modification]
    2: the act of making something different (as e.g. the size of a
       garment) [syn: alteration, modification, adjustment]
    3: the act of revising or altering (involving reconsideration
       and modification); "it would require a drastic revision of
       his opinion" [syn: revision, alteration]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "alteration":
   about-face, accommodation, adaptation, adjustment, amelioration,
   analysis, anatomization, apostasy, atomization, betterment, break,
   change, change of heart, changeableness, changeover,
   constructive change, continuity, conversion, defection,
   degeneration, degenerative change, demarcation, desynonymization,
   deterioration, deviation, difference, differencing,
   differentiation, discontinuity, discrimination, disequalization,
   disjunction, distinction, distinguishment, divergence,
   diversification, diversion, diversity, division, fitting,
   flip-flop, fluctuation, gradual change, improvement,
   individualization, individuation, melioration, metamorphosis,
   mitigation, modification, modulation, mutation, overthrow,
   particularization, passage, personalization, qualification,
   radical change, re-creation, realignment, redesign, reform,
   reformation, remaking, renewal, reshaping, restructuring, reversal,
   revision, revival, revivification, revolution, segregation,
   separation, severalization, severance, shift, shilly-shally,
   specialization, sudden change, switch, total change,
   transformation, transit, transition, turn, turnabout, upheaval,
   vacillation, variation, variety, violent change, wavering,
   worsening