The Collaborative International Dictionary of English v.0.48:

Affirmation \Af`fir*ma"tion\, n. [L. affirmatio: cf. F.
   affirmation.]
   1. Confirmation of anything established; ratification; as,
      the affirmation of a law. --Hooker.
      [1913 Webster]

   2. The act of affirming or asserting as true; assertion; --
      opposed to negation or denial.
      [1913 Webster]

   3. That which is asserted; an assertion; a positive
      statement; an averment; as, an affirmation, by the vender,
      of title to property sold, or of its quality.
      [1913 Webster]

   4. (Law) A solemn declaration made under the penalties of
      perjury, by persons who conscientiously decline taking an
      oath, which declaration is in law equivalent to an oath.
      --Bouvier.
      [1913 Webster]

WordNet (r) 3.0 (2006):

affirmation
    n 1: a statement asserting the existence or the truth of
         something [syn: avowal, avouchment, affirmation]
    2: the act of affirming or asserting or stating something [syn:
       affirmation, assertion, statement]
    3: (religion) a solemn declaration that serves the same purpose
       as an oath (if an oath is objectionable to the person on
       religious or ethical grounds)
    4: a judgment by a higher court that the judgment of a lower
       court was correct and should stand [ant: reversal]