The Collaborative International Dictionary of English v.0.48:

Naturalize \Nat"u*ral*ize\ (?; 135), v. t. [imp. & p. p.
   Naturalized; p. pr. & vb. n. Naturalizing.] [Cf. F.
   naturaliser. See Natural.]
   1. To make natural; as, custom naturalizes labor or study.
      [1913 Webster]

   2. To confer the rights and privileges of a native subject or
      citizen on; to make as if native; to adopt, as a foreigner
      into a nation or state, and place in the condition of a
      native subject.
      [1913 Webster]

   3. To receive or adopt as native, natural, or vernacular; to
      make one's own; as, to naturalize foreign words.
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   4. To adapt; to accustom; to habituate; to acclimate; to
      cause to grow as under natural conditions.
      [1913 Webster]

            Its wearer suggested that pears and peaches might
            yet be naturalized in the New England climate.
                                                  --Hawthorne.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Naturalize \Nat"u*ral*ize\, v. i.
   1. To become as if native.
      [1913 Webster]

   2. To explain phenomena by natural agencies or laws, to the
      exclusion of the supernatural.
      [1913 Webster]

            Infected by this naturalizing tendency. --H.
                                                  Bushnell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

naturalize
    v 1: make into a citizen; "The French family was naturalized
         last year" [syn: naturalize, naturalise] [ant:
         denaturalise, denaturalize]
    2: explain with reference to nature
    3: adopt to another place; "The stories had become naturalized
       into an American setting" [syn: naturalize, naturalise]
    4: make more natural or lifelike [syn: naturalize,
       naturalise] [ant: denaturalise, denaturalize]
    5: adapt (a wild plant or unclaimed land) to the environment;
       "domesticate oats"; "tame the soil" [syn: domesticate,
       cultivate, naturalize, naturalise, tame]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "naturalize":
   Americanize, Anglicize, acclimate, acclimatize, accommodate,
   acculturate, acculturize, accustom, adapt, adjust, admit, adopt,
   affiliate, assimilate, assimilate to, become, break, break in,
   bring to, case harden, change, change into, change over, condition,
   confer citizenship, confirm, convert, do over, domesticate,
   domesticize, establish, familiarize, fix, gentle, go native,
   habituate, harden, housebreak, inure, make, make over, orient,
   orientate, reconvert, reduce to, render, resolve into, reverse,
   season, shift, switch, switch over, tame, train, transform,
   turn back, turn into, wont