Source : Webster's Revised Unabridged Dictionary (1913)

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

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

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

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.

   2. To assert positively; to tell with confidence; to aver; to
      maintain as true; -- opposed to {deny}.

            Jesus, . . . whom Paul affirmed to be alive. --Acts
                                                  xxv. 19.

   3. (Law) To declare, as a fact, solemnly, under judicial
      sanction. See {Affirmation}, 4.

   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.

Source : WordNet®

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}] [ant: {negate}]
     2: to declare or affirm solemnly and formally as true; "Before
        God I swear I am innocent" [syn: {verify}, {assert}, {avow},
         {aver}, {swan}, {swear}]
     3: say yes to