The Collaborative International Dictionary of English v.0.48:

Justify \Jus"ti*fy\, v. t. [imp. & p. p. Justified; p. pr. &
   vb. n. Justifying.] [F. justifier, L. justificare; justus
   just + -ficare (in comp.) to make. See Just, a., and
   -fy.]
   [1913 Webster]
   1. To prove or show to be just; to vindicate; to maintain or
      defend as conformable to law, right, justice, propriety,
      or duty.
      [1913 Webster]

            That to the height of this great argument
            I may assert eternal providence,
            And justify the ways of God to men.   --Milton.
      [1913 Webster]

            Unless the oppression is so extreme as to justify
            revolution, it would not justify the evil of
            breaking up a government.             --E. Everett.
      [1913 Webster]

   2. To pronounce free from guilt or blame; to declare or prove
      to have done that which is just, right, proper, etc.; to
      absolve; to exonerate; to clear.
      [1913 Webster]

            I can not justify whom the law condemns. --Shak.
      [1913 Webster]

   3. (Theol.) To treat as if righteous and just; to pardon; to
      exculpate; to absolve.
      [1913 Webster]

            By him all that believe are justified from all
            things, from which ye could not be justified by the
            law of Moses.                         --Acts xiii.
                                                  39.
      [1913 Webster]

   4. To prove; to ratify; to confirm. [Obs.] --Shak.
      [1913 Webster]

   5. (Print.) To make even or true, as lines of type, by proper
      spacing; to align (text) at the left (left justify) or
      right (right justify) margins of a column or page, or at
      both margins; to adjust, as type. See Justification, 4.
      [1913 Webster]

   6. (Law)
      (a) To show (a person) to have had a sufficient legal
          reason for an act that has been made the subject of a
          charge or accusation.
      (b) To qualify (one's self) as a surety by taking oath to
          the ownership of sufficient property.
          [Webster 1913 Suppl.]

                The production of bail in court, who there
                justify themselves against the exception of the
                plaintiff.                        --Bouvier's
                                                  Law Dict.
          [Webster 1913 Suppl.]

   Syn: To defend; maintain; vindicate; excuse; exculpate;
        absolve; exonerate.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Justify \Jus"ti*fy\, v. i.
   1. (Print.) To form an even surface or true line with
      something else; to fit exactly.
      [1913 Webster]

   2. (Law) To take oath to the ownership of property sufficient
      to qualify one's self as bail or surety.
      [1913 Webster]

WordNet (r) 3.0 (2006):

justify
    v 1: show to be reasonable or provide adequate ground for; "The
         emergency does not warrant all of us buying guns"; "The end
         justifies the means" [syn: justify, warrant]
    2: show to be right by providing justification or proof;
       "vindicate a claim" [syn: justify, vindicate]
    3: defend, explain, clear away, or make excuses for by
       reasoning; "rationalize the child's seemingly crazy
       behavior"; "he rationalized his lack of success" [syn:
       apologize, apologise, excuse, justify, rationalize,
       rationalise]
    4: let off the hook; "I absolve you from this responsibility"
       [syn: absolve, justify, free] [ant: blame, fault]
    5: adjust the spaces between words; "justify the margins"

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "justify":
   absolve, account, account for, acquit, allow, amnesty, approve,
   argue, assert, authenticate, authorize, back, bear out, claim,
   clear, compose, confirm, contend, corroborate, cry sour grapes,
   decontaminate, defend, demonstrate, destigmatize, discharge,
   dismiss, dispense from, do justice to, exculpate, excuse, exempt,
   exempt from, exonerate, explain, explain away, extenuate, forgive,
   free, give absolution, gloss, grant amnesty to, grant immunity,
   grant remission, impose, legalize, legitimate, legitimatize,
   legitimize, let go, let off, make up, nonpros, overrun, palliate,
   pardon, permit, pi, pi a form, prove, purge, quash the charge,
   rationalize, rehabilitate, reinstate, release, remit, restore,
   sanction, set, set free, set in print, shrive, substantiate,
   support, sustain, uphold, validate, verify, vindicate, warrant,
   whitewash, withdraw the charge