The Collaborative International Dictionary of English v.0.48:

Authorize \Au"thor*ize\, v. t. [imp. & p. p. Authorized; p.
   pr. & vb. n. Authorizing.] [OE. autorize, F. autoriser, fr.
   LL. auctorizare, authorisare. See Author.]
   1. To clothe with authority, warrant, or legal power; to give
      a right to act; to empower; as, to authorize commissioners
      to settle a boundary.
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   2. To make legal; to give legal sanction to; to legalize; as,
      to authorize a marriage.
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   3. To establish by authority, as by usage or public opinion;
      to sanction; as, idioms authorized by usage.
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   4. To sanction or confirm by the authority of some one; to
      warrant; as, to authorize a report.
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            A woman's story at a winter's fire,
            Authorized by her grandam.            --Shak.
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   5. To justify; to furnish a ground for. --Locke.
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   To authorize one's self, to rely for authority. [Obs.]
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            Authorizing himself, for the most part, upon other
            histories.                            --Sir P.
                                                  Sidney.
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WordNet (r) 3.0 (2006):

authorize
    v 1: grant authorization or clearance for; "Clear the manuscript
         for publication"; "The rock star never authorized this
         slanderous biography" [syn: authorize, authorise,
         pass, clear]
    2: give or delegate power or authority to; "She authorized her
       assistant to sign the papers" [syn: empower, authorise,
       authorize]

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "authorize":
   OK, accept, accredit, affirm, allow, amen, appoint, approve, arm,
   assign, authenticate, autograph, certificate, certify, charge,
   charter, clothe, clothe with power, commission, commit, confirm,
   consent to, consign, constitute, cosign, countenance, countersign,
   declare lawful, decree, delegate, demand, depute, deputize, detach,
   detail, devolute, devolve, devolve upon, dictate, empower, enable,
   enact, endorse, endow, endue, enfranchise, entitle, entrust,
   establish, formulate, franchise, give in charge, give leave,
   give official sanction, give permission, give power,
   give the go-ahead, give the imprimatur, give thumbs up, impose,
   initial, invest, lay down, legalize, legislate, legitimate,
   legitimatize, legitimize, license, make a regulation, make legal,
   make obligatory, mission, notarize, okay, ordain, pass, pass on,
   pass upon, patent, permit, post, prescribe, privilege,
   put in force, ratify, regulate, require, rubber stamp, sanction,
   say amen to, seal, second, send out, set, sign, sign and seal,
   subscribe to, support, swear and affirm, swear to, transfer,
   undersign, underwrite, validate, visa, vise, warrant