The Collaborative International Dictionary of English v.0.48:

Affirmative \Af*firm"a*tive\, a. [L. affirmativus: cf. F.
   affirmatif.]
   1. Confirmative; ratifying; as, an act affirmative of common
      law.
      [1913 Webster]

   2. That affirms; asserting that the fact is so; declaratory
      of what exists; answering "yes" to a question; -- opposed
      to negative; as, an affirmative answer; an affirmative
      vote.
      [1913 Webster]

   3. Positive; dogmatic. [Obs.] --J. Taylor.
      [1913 Webster]

            Lysicles was a little by the affirmative air of
            Crito.                                --Berkeley.
      [1913 Webster]

   4. (logic) Expressing the agreement of the two terms of a
      proposition.
      [1913 Webster]

   5. (Alg.) Positive; -- a term applied to quantities which are
      to be added, and opposed to negative, or such as are to
      be subtracted.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Affirmative \Af*firm"a*tive\, n.
   1. That which affirms as opposed to that which denies; an
      affirmative proposition; that side of question which
      affirms or maintains the proposition stated; -- opposed to
      negative; as, there were forty votes in the affirmative,
      and ten in the negative.
      [1913 Webster]

            Whether there are such beings or not, 't is
            sufficient for my purpose that many have believed
            the affirmative.                      --Dryden.
      [1913 Webster]

   2. A word or phrase expressing affirmation or assent; as,
      yes, that is so, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

affirmative
    adj 1: affirming or giving assent; "an affirmative decision";
           "affirmative votes" [syn: affirmative, affirmatory]
           [ant: negative]
    2: expecting the best; "an affirmative outlook" [syn:
       affirmative, optimistic]
    3: expressing or manifesting praise or approval; "approbative
       criticism"; "an affirmative nod" [syn: approving,
       affirmative, approbative, approbatory, plausive]
    n 1: a reply of affirmation; "he answered in the affirmative"
         [ant: negative]

Moby Thesaurus II by Grady Ward, 1.0:

132 Moby Thesaurus words for "affirmative":
   OK, absolute, acceptance, accord, accordant, acquiescence,
   acquiescent, affirmation, affirmative attitude, affirmative voice,
   affirmativeness, affirmatory, agree, agreeable, agreeing,
   agreement, amen, answerable, approbation, approval, approving,
   assent, assenting, assertative, assertional, assertive, at one,
   aye, blessing, coexistent, coexisting, coherent, coincident,
   coinciding, commensurate, compatible, compliable, compliance,
   compliant, con, concordant, concurring, conformable, congenial,
   congruent, congruous, connivance, consent, consentaneous,
   consentient, consenting, consistent, consonant, content,
   cooperating, cooperative, correspondent, corresponding, decided,
   declarative, declaratory, eager, eagerness, emphatic, en rapport,
   endorsement, endorsing, equivalent, favorable, harmonious,
   in accord, in agreement, in rapport, in sync, in synchronization,
   inaccordance, inharmony, interest, like-minded, nay, no, nod,
   nod of assent, nothing loath, of a piece, of like mind,
   of one mind, okay, on all fours, permission, permissive, positive,
   predicational, predicative, pro, prompt, promptitude, promptness,
   proportionate, ratification, ratifying, readiness, ready,
   reconcilable, sanction, sanctioning, self-consistent, side,
   submission, submissive, symbiotic, synchronized, synchronous,
   the affirmative, the negative, thumbs-up, unanimous, ungrudging,
   ungrudgingness, uniform, unisonant, unisonous, unloath,
   unloathness, unrefusing, unreluctance, unreluctant, willing,
   willingness, yea, yea-saying, yes



Bouvier's Law Dictionary, Revised 6th Ed (1856):

AFFIRMATIVE. Averring a fact to be true; that which is opposed to negative.
(q.v.)
     2. It is a general rule of evidence that the affirmative of the issue
must be proved. Bull. N. P. 298 ; Peake, Ev. 2.
     3. But when the law requires a person to do an act, and the neglect of
it, will render him guilty and punishable, the negative must be proved,
because every man is presumed to do his duty and in that case they who
affirm he did not, must prove it. B. N. P. 298; 1 Roll. R. 83; Comb. 57; 3
B.& P. 307; 1 Mass. R. 56.