The Collaborative International Dictionary of English v.0.48:

Assimilate \As*sim"i*late\, v. i.
   1. To become similar or like something else. [R.]
      [1913 Webster]

   2. To change and appropriate nourishment so as to make it a
      part of the substance of the assimilating body.
      [1913 Webster]

            Aliment easily assimilated or turned into blood.
                                                  --Arbuthnot.
      [1913 Webster]

   3. To be converted into the substance of the assimilating
      body; to become incorporated; as, some kinds of food
      assimilate more readily than others.
      [1913 Webster]

            I am a foreign material, and cannot assimilate with
            the church of England.                --J. H.
                                                  Newman.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Assimilate \As*sim"i*late\, v. t. [imp. & p. p. Assimilated;
   p. pr. & vb. n. Assimilating.] [L. assimilatus, p. p. of
   assimilare; ad + similare to make like, similis like. See
   Similar, Assemble, Assimilate.]
   1. To bring to a likeness or to conformity; to cause a
      resemblance between. --Sir M. Hale.
      [1913 Webster]

            To assimilate our law to the law of Scotland. --John
                                                  Bright.
      [1913 Webster]

            Fast falls a fleecy; the downy flakes
            Assimilate all objects.               --Cowper.
      [1913 Webster]

   2. To liken; to compa?e. [R.]
      [1913 Webster]

   3. To appropriate and transform or incorporate into the
      substance of the assimilating body; to absorb or
      appropriate, as nourishment; as, food is assimilated and
      converted into organic tissue.
      [1913 Webster]

            Hence also animals and vegetables may assimilate
            their nourishment.                    --Sir I.
                                                  Newton.
      [1913 Webster]

            His mind had no power to assimilate the lessons.
                                                  --Merivale.
      [1913 Webster]

WordNet (r) 3.0 (2006):

assimilate
    v 1: take up mentally; "he absorbed the knowledge or beliefs of
         his tribe" [syn: absorb, assimilate, ingest, take
         in]
    2: become similar to one's environment; "Immigrants often want
       to assimilate quickly" [ant: dissimilate]
    3: make similar; "This country assimilates immigrants very
       quickly" [ant: dissimilate]
    4: take (gas, light or heat) into a solution [syn: assimilate,
       imbibe]
    5: become similar in sound; "The nasal assimilates to the
       following consonant" [ant: dissimilate]

Moby Thesaurus II by Grady Ward, 1.0:

290 Moby Thesaurus words for "assimilate":
   Americanize, Anglicize, ablate, absorb, accommodate,
   accommodate with, accord, acculturate, acculturize, adapt,
   adapt to, add, adjust, adjust to, admit, adopt, adsorb, affiliate,
   agree with, amalgamate, analogize, appreciate, apprehend,
   appropriate, approximate, assimilate, assimilate to, attune,
   balance, be guided by, be with one, become, bend, bleed white,
   blend, blot, blot up, bring into analogy, bring into comparison,
   bring near, bring to, burn up, catch, catch on, change,
   change into, change over, chemisorb, chemosorb, chime in with,
   coalesce, combine, come together, compare, compare and contrast,
   compare with, complete, comply, comply with, compose, compound,
   comprehend, comprise, conceive, confer citizenship, conform,
   confront, connaturalize, connect, consolidate, consume, contain,
   contrast, convert, coordinate, corner, correct, correspond,
   count in, counterpose, cover, cut to, damp, deplete, dig, digest,
   discipline, do over, drain, drain of resources, draw a comparison,
   draw a parallel, drink, drink in, drink up, eat, eat up, embody,
   embrace, encircle, enclose, encompass, engross, envisage, equalize,
   equilibrize, erode, espouse, even, exhaust, expend, fall in with,
   fathom, fill, fill in, fill out, filter in, finish, finish off,
   fit, fix, flatten, flux, follow, fuse, gear to, get, get hold of,
   get the drift, get the idea, get the picture, go by, go native,
   gobble, gobble up, grasp, harmonize, have, have it taped, hold,
   homogenize, homologate, homologize, imbibe, imbue, impoverish,
   include, incorporate, infiltrate, infuse, ingest, ingrain,
   inoculate, integrate, interblend, interfuse, join, ken, key to,
   know, learn, leaven, level, liken, liken to, lump together, make,
   make conform, make one, make over, make plumb, make uniform,
   master, match, measure, measure against, meet, meld, melt into one,
   merge, metabolize, metaphorize, mix, mold, monopolize, naturalize,
   normalize, number among, observe, occupy, oppose, osmose, paragon,
   parallel, percolate in, place against, predigest, proportion,
   put in tune, put together, read, realize, receive, reckon among,
   reckon in, reckon with, reconcile, reconvert, rectify, reduce to,
   reembody, regularize, regulate, relate, render, resolve into,
   reverse, right, roll into one, rub off corners, run a comparison,
   savvy, seep in, seize, seize the meaning, sense, set,
   set in contrast, set in opposition, set off against,
   set over against, set right, settle, shade into, shape, shift,
   similarize, similize, slurp up, smooth, soak in, soak up, solidify,
   sorb, spend, sponge, squander, stabilize, standardize, stereotype,
   straighten, suck dry, suffuse, suit, swallow, swallow up, swill up,
   switch, switch over, symmetrize, sync, synchronize, syncretize,
   syndicate, synthesize, tailor, take, take in, take into account,
   take into consideration, take up, tally with, transform, trim to,
   true, true up, tune, turn back, turn into, understand, uniformize,
   unify, unite, use up, view together, waste away, wear away, weigh,
   weigh against, yield