The Collaborative International Dictionary of English v.0.48:

Differentiation \Dif`fer*en`ti*a"tion\, n.
   1. The act of differentiating.
      [1913 Webster]

            Further investigation of the Sanskrit may lead to
            differentiation of the meaning of such of these
            roots as are real roots.              --J. Peile.
      [1913 Webster]

   2. (Logic) The act of distinguishing or describing a thing,
      by giving its different, or specific difference; exact
      definition or determination.
      [1913 Webster]

   3. (Biol.) The gradual formation or production of organs or
      parts by a process of evolution or development, as when
      the seed develops the root and the stem, the initial stem
      develops the leaf, branches, and flower buds; or in animal
      life, when the germ evolves the digestive and other organs
      and members, or when the animals as they advance in
      organization acquire special organs for specific purposes.
      [1913 Webster]

   4. (Metaph.) The supposed act or tendency in being of every
      kind, whether organic or inorganic, to assume or produce a
      more complex structure or functions.
      [1913 Webster]

WordNet (r) 3.0 (2006):

differentiation
    n 1: a discrimination between things as different and distinct;
         "it is necessary to make a distinction between love and
         infatuation" [syn: differentiation, distinction]
    2: the mathematical process of obtaining the derivative of a
       function
    3: (biology) the structural adaptation of some body part for a
       particular function; "cell differentiation in the developing
       embryo" [syn: specialization, specialisation,
       differentiation]

Moby Thesaurus II by Grady Ward, 1.0:

135 Moby Thesaurus words for "differentiation":
   addition, alteration, analysis, anatomization, approximation,
   atomization, capriciousness, change, changeability, changeableness,
   characterization, choppiness, contradistinction, dappleness,
   definition, demarcation, denomination, denotation, description,
   designation, desynonymization, deviation, difference, differencing,
   differentness, disclosure, discrimination, disequalization,
   disjunction, disorder, distinction, distinctiveness,
   distinguishment, divarication, divergence, diversification,
   division, egohood, equation, evolution, expression, extrapolation,
   fingering, hairsplitting, hint, human factor, identification,
   identity, inconsistency, inconstancy, indication, indicativeness,
   individualism, individuality, individualization, individuation,
   inequality, instability, integration, integrity, interpolation,
   inversion, involution, irregularity, jerkiness, manifestation,
   meaning, mercuriality, microscopic distinction, modification,
   motleyness, multiplication, mutability, naming, nice distinction,
   nominalism, nonconformism, nonconformity, nonstandardization,
   nonuniformity, notation, nuance, oneness, particularism,
   particularity, particularization, personal equation,
   personal identity, personality, personalization, personship,
   picking out, pluralism, pointing, pointing out, pointing to,
   practice, proportion, raggedness, reduction, segregation,
   selection, self-identity, selfhood, selfness, separation,
   severalization, severance, shade of difference, show, showing,
   signification, singularity, soul, specialization, specification,
   subtle distinction, subtraction, suggestion, symptomaticness,
   transformation, trichoschistism, unconformism, unconformity,
   unevenness, uniqueness, unorthodoxy, unsteadiness, variability,
   variation, variegation, variety, variousness, versatility,
   wavering