The Collaborative International Dictionary of English v.0.48:

Differentiate \Dif`fer*en"ti*ate\, v. t.
   1. To distinguish or mark by a specific difference; to effect
      a difference in, as regards classification; to develop
      differential characteristics in; to specialize; to
      desynonymize.
      [1913 Webster]

            The word then was differentiated into the two forms
            then and than.                        --Earle.
      [1913 Webster]

            Two or more of the forms assumed by the same
            original word become differentiated in
            signification.                        --Dr. Murray.
      [1913 Webster]

   2. To express the specific difference of; to describe the
      properties of (a thing) whereby it is differenced from
      another of the same class; to discriminate. --Earle.
      [1913 Webster]

   3. (Math.) To obtain the differential, or differential
      coefficient, of; as, to differentiate an algebraic
      expression, or an equation.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Differentiate \Dif`fer*en"ti*ate\, v. i. (Biol.)
   To acquire a distinct and separate character. --Huxley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

differentiate
    v 1: mark as different; "We distinguish several kinds of maple"
         [syn: distinguish, separate, differentiate, secern,
         secernate, severalize, severalise, tell, tell
         apart]
    2: be a distinctive feature, attribute, or trait; sometimes in a
       very positive sense; "His modesty distinguishes him from his
       peers" [syn: distinguish, mark, differentiate]
    3: calculate a derivative; take the derivative [ant:
       integrate]
    4: become different during development; "cells differentiate"
       [ant: dedifferentiate]
    5: evolve so as to lead to a new species or develop in a way
       most suited to the environment [syn: speciate,
       differentiate, specialize, specialise]
    6: become distinct and acquire a different character

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "differentiate":
   adapt, adjust, alter, analyze, anatomize, argue, atomize,
   be characteristic, be indicative of, be significant of,
   be symptomatic of, bespeak, betoken, break up, calibrate,
   calibrated, change, characterize, chop logic, comprehend, connote,
   contradistinguish, contrast, convert, decrease, define, demarcate,
   demark, denominate, denote, describe, desynonymize, differ,
   difference, discern, disclose, discriminate, disequalize, disjoin,
   display, dissent, distinguish, disunify, divaricate, diverge,
   diversify, divide, draw the line, earmark, entail, express,
   extricate, give evidence, give token, gradational, grade, gradual,
   graduate, graduated, hierarchic, highlight, hint, identify,
   increase, indicate, individualize, individuate, involve, keynote,
   make a distinction, manifest, mark, mark off, mark out,
   mark the interface, mean, modify, note, oppose, particularize,
   personalize, pick out, progressive, refine a distinction, regular,
   relieve, reveal, scalar, screen, screen out, segregate, select,
   separate, set a limit, set apart, set off, set the pace,
   set the tone, sever, severalize, shade off, show, sieve, sieve out,
   sift, sift out, signify, sort, sort out, sound the keynote,
   specialize, split hairs, stand for, subdivide, subtilize, suggest,
   symptomatize, symptomize, tell apart, testify, transform,
   transmute, understand, variate, variegate, vary, winnow