The Collaborative International Dictionary of English v.0.48:

Approximation \Ap*prox`i*ma"tion\ n. [Cf. F. approximation, LL.
   approximatio.]
   1. The act of approximating; a drawing, advancing or being
      near; approach; also, the result of approximating.
      [1913 Webster]

            The largest capacity and the most noble dispositions
            are but an approximation to the proper standard and
            true symmetry of human nature.        --I. Taylor.
      [1913 Webster]

   2. An approach to a correct estimate, calculation, or
      conception, or to a given quantity, quality, etc.
      [1913 Webster]

   3. (Math.)
      (a) A continual approach or coming nearer to a result; as,
          to solve an equation by approximation.
      (b) A value that is nearly but not exactly correct.
          [1913 Webster]

WordNet (r) 3.0 (2006):

approximation
    n 1: an approximate calculation of quantity or degree or worth;
         "an estimate of what it would cost"; "a rough idea how long
         it would take" [syn: estimate, estimation,
         approximation, idea]
    2: the quality of coming near to identity (especially close in
       quantity)
    3: an imprecise or incomplete account; "newspapers gave only an
       approximation of the actual events"
    4: the act of bringing near or bringing together especially the
       cut edges of tissue [syn: approximation, bringing close
       together]

Moby Thesaurus II by Grady Ward, 1.0:

162 Moby Thesaurus words for "approximation":
   access, accession, accord, accordance, addition, adjunct, advance,
   advent, affairs, affiliation, affinity, afflux, affluxion,
   agreement, alikeness, alliance, allowance, analogy, aping,
   appraisal, appraisement, approach, approaching, appropinquation,
   appulse, assemblage, assessment, assimilation, assize, assizement,
   association, bond, calculation, closeness, combination, coming,
   coming near, coming toward, community, comparability, comparison,
   computation, confines, conformity, connectedness, connection,
   contiguity, contrariety, convergence, copying, correction,
   correspondence, dealings, deduction, determination, deviation,
   differentiation, disjunction, division, environs, equation,
   estimate, estimation, evaluation, evolution, extrapolation,
   filiation, flowing toward, foreground, forthcoming, gauging,
   homology, identity, imitation, immediacy, immediate foreground,
   imminence, imprecision, inaccuracy, inaccurateness, incorrectness,
   inexactitude, inexactness, instrumentation, integration,
   intercourse, interpolation, intimacy, inversion, involution,
   junction, laxity, liaison, likeness, likening, link, linkage,
   linking, looseness, measure, measurement, measuring, mensuration,
   metaphor, metric system, mimicking, multiplication,
   mutual attraction, nearing, nearness, negligence, neighborhood,
   nighness, notation, oncoming, parallelism, parity, practice,
   precinct, predictable error, probable error, propinquity,
   proportion, proximation, proximity, purlieus, quantification,
   quantization, rapport, rating, reduction, relatedness, relation,
   relations, relationship, resemblance, sameness, semblance,
   similarity, simile, similitude, simulation, standard deviation,
   subtraction, survey, surveying, sympathy, telemetering, telemetry,
   tie, tie-in, tolerance, transformation, triangulation,
   uncorrectness, unfactualness, union, unpreciseness, unrigorousness,
   valuation, vicinage, vicinity