The Collaborative International Dictionary of English v.0.48:

Naturalization \Nat`u*ral*i*za"tion\, n. [Cf. F.
   naturalisation.]
   The act or process of naturalizing, esp. of investing an
   alien with the rights and privileges of a native or citizen;
   also, the state of being naturalized.
   [1913 Webster]

WordNet (r) 3.0 (2006):

naturalization
    n 1: the quality of being brought into conformity with nature
         [syn: naturalization, naturalisation]
    2: the proceeding whereby a foreigner is granted citizenship
       [syn: naturalization, naturalisation]
    3: the introduction of animals or plants to places where they
       flourish but are not indigenous [syn: naturalization,
       naturalisation]
    4: changing the pronunciation of a borrowed word to agree with
       the borrowers' phonology; "the naturalization in English of
       many Italian words" [syn: naturalization, naturalisation]