The Collaborative International Dictionary of English v.0.48:

Legalize \Le"gal*ize\ (l[=e]"gal*[imac]z), v. t. [imp. & p. p.
   Legalized (-[imac]zd); p. pr. & vb. n. Legalizing
   (-[imac]`z[i^]ng).] [Cf. F. l['e]galiser.]
   1. To make legal.
      [1913 Webster]

   2. (Theol.) To interpret or apply in a legal spirit.
      [1913 Webster]

WordNet (r) 3.0 (2006):

legalize
    v 1: make legal; "Marijuana should be legalized" [syn:
         legalize, legalise, decriminalize, decriminalise,
         legitimize, legitimise, legitimate, legitimatize,
         legitimatise] [ant: criminalise, criminalize,
         illegalise, illegalize, outlaw]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "legalize":
   authorize, certificate, certify, charter, constitute,
   declare lawful, decree, empower, enable, enact, enfranchise,
   entitle, establish, formulate, franchise, give official sanction,
   give power, legislate, legitimate, legitimatize, legitimize,
   license, make a regulation, make legal, ordain, patent, prescribe,
   privilege, put in force, ratify, regulate, sanction, validate,
   warrant