The Collaborative International Dictionary of English v.0.48:

Affirm \Af*firm"\ ([a^]f*f[~e]rm"), v. t. [imp. & p. p.
   Affirmed (-f[~e]rmd"); p. pr. & vb. n. Affirming.] [OE.
   affermen, OF. afermer, F. affirmer, affermir, fr. L.
   affirmare; ad + firmare to make firm, firmus firm. See
   Firm.]
   1. To make firm; to confirm, or ratify; esp. (Law), to assert
      or confirm, as a judgment, decree, or order, brought
      before an appellate court for review.
      [1913 Webster]

   2. To assert positively; to tell with confidence; to aver; to
      maintain as true; -- opposed to deny.
      [1913 Webster]

            Jesus, . . . whom Paul affirmed to be alive. --Acts
                                                  xxv. 19.
      [1913 Webster]

   3. (Law) To declare, as a fact, solemnly, under judicial
      sanction. See Affirmation, 4.
      [1913 Webster]

   Syn: To assert; aver; declare; asseverate; assure; pronounce;
        protest; avouch; confirm; establish; ratify.

   Usage: To Affirm, Asseverate, Aver, Protest. We
          affirm when we declare a thing as a fact or a
          proposition. We asseverate it in a peculiarly earnest
          manner, or with increased positiveness as what can not
          be disputed. We aver it, or formally declare it to be
          true, when we have positive knowledge of it. We
          protest in a more public manner and with the energy of
          perfect sincerity. People asseverate in order to
          produce a conviction of their veracity; they aver when
          they are peculiarly desirous to be believed; they
          protest when they wish to free themselves from
          imputations, or to produce a conviction of their
          innocence.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Affirm \Af*firm"\, v. i.
   1. To declare or assert positively.
      [1913 Webster]

            Not that I so affirm, though so it seem
            To thee, who hast thy dwelling here on earth.
                                                  --Milton.
      [1913 Webster]

   2. (Law) To make a solemn declaration, before an authorized
      magistrate or tribunal, under the penalties of perjury; to
      testify by affirmation.
      [1913 Webster]

WordNet (r) 3.0 (2006):

affirm
    v 1: establish or strengthen as with new evidence or facts; "his
         story confirmed my doubts"; "The evidence supports the
         defendant" [syn: confirm, corroborate, sustain,
         substantiate, support, affirm] [ant: contradict,
         negate]
    2: to declare or affirm solemnly and formally as true; "Before
       God I swear I am innocent" [syn: affirm, verify,
       assert, avow, aver, swan, swear]
    3: say yes to

Moby Thesaurus II by Grady Ward, 1.0:

122 Moby Thesaurus words for "affirm":
   OK, accept, accredit, acknowledge, allege, amen, announce,
   annunciate, approve, argue, assert, assever, asseverate, attest,
   authenticate, authorize, autograph, aver, avouch, avow, back,
   back up, bear out, bear witness, bolster, buttress, certify,
   circumstantiate, confess, confirm, contend, corroborate, cosign,
   countersign, declare, declare roundly, depone, depose, disclose,
   document, endorse, enunciate, express, express the belief, fortify,
   give evidence, give notice, give permission, give the go-ahead,
   give the imprimatur, give thumbs up, guarantee, have, hold,
   initial, insist, issue a manifesto, issue a statement, lay down,
   maintain, make a statement, make an announcement, manifesto,
   notarize, nuncupate, pass, pass on, pass upon, permit, predicate,
   probate, proclaim, profess, pronounce, protest, prove,
   publish a manifesto, put, put it, quote, ratify, recite, reinforce,
   relate, report, rubber stamp, sanction, say, say amen to, seal,
   second, set down, sign, sign and seal, speak, speak out, speak up,
   stand for, stand on, state, strengthen, submit, subscribe to,
   substantiate, support, sustain, swear, swear and affirm, swear to,
   testify, undergird, undersign, underwrite, uphold, validate,
   verify, visa, vise, vouch, vow, warrant, witness