The Collaborative International Dictionary of English v.0.48:

naturalized \naturalized\ adj.
   1. Acclimated to a new environment; introduced from another
      region and persisting without cultivation; -- of plants or
      animals not native to a location. [WordNet sense 1 & 3]

   Syn: domesticated, nonnative.
        [WordNet 1.5]

   2. planted randomly in soil so as to give an appearance of
      wild growth; as, drifts of naturalized daffodils.
      [WordNet 1.5 +PJC]

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.
      [1913 Webster]

   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]

WordNet (r) 3.0 (2006):

naturalized
    adj 1: introduced from another region and persisting without
           cultivation [syn: established, naturalized]
    2: planted so as to give an effect of wild growth; "drifts of
       naturalized daffodils" [syn: naturalized, naturalised]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "naturalized":
   Americanized, Anglicized, acclimated, acclimatized, accommodated,
   acculturated, acculturized, accustomed, adapted, adjusted, adopted,
   assimilated, case-hardened, changed, conditioned, converted,
   experienced, familiarized, hardened, indoctrinated, inured,
   orientated, oriented, reborn, redeemed, reformed, regenerated,
   renewed, run-in, seasoned, trained, transformed, used to, wont,
   wonted