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]

Moby Thesaurus II by Grady Ward, 1.0:

162 Moby Thesaurus words for "affirmation":
   John Hancock, OK, Parthian shot, a priori principle, acceptance,
   accord, acquiescence, address, admission, affidavit, affirmance,
   affirmative, affirmative voice, agreement, allegation, answer,
   apostrophe, approbation, approval, apriorism, assent, assertion,
   asseveration, assumed position, assumption, attest, attestation,
   authentication, authorization, averment, avouchment, avowal, axiom,
   aye, backing, backing up, basis, bearing out, blessing, bolstering,
   buttressing, categorical proposition, certification,
   circumstantiation, comment, compliance, compurgation, confirmation,
   connivance, consent, corroboration, corroboratory evidence,
   countersignature, crack, data, declaration, deposition, dictum,
   disclosure, documentation, eagerness, endorsement, exclamation,
   expression, first principles, fortification, foundation, go-ahead,
   green light, greeting, ground, hypothesis, hypothesis ad hoc,
   imprimatur, instrument in proof, interjection, legal evidence,
   lemma, major premise, mention, minor premise, nod, notarization,
   notarized statement, note, observation, okay, permission,
   philosopheme, philosophical proposition, phrase, position,
   postulate, postulation, postulatum, premise, presupposition,
   profession, promptitude, promptness, pronouncement, proof,
   proposition, propositional function, proving, proving out,
   question, ratification, readiness, reflection, reinforcement,
   remark, rubber stamp, sanction, say, saying, seal, sentence, sigil,
   signature, signet, stamp, stamp of approval, statement,
   statement under oath, strengthening, subjoinder, submission,
   subscription, substantiation, sumption, support,
   supporting evidence, supposal, swearing, sworn evidence,
   sworn statement, sworn testimony, testimonial, testimonium,
   testimony, the nod, theorem, thesis, thought, truth table,
   truth-function, truth-value, undergirding, ungrudgingness,
   unloathness, unreluctance, utterance, validation, verification,
   visa, vise, vouching, warrant, willingness, witness, word