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.
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   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.
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   3. Positive; dogmatic. [Obs.] --J. Taylor.
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            Lysicles was a little by the affirmative air of
            Crito.                                --Berkeley.
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   4. (logic) Expressing the agreement of the two terms of a
      proposition.
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   5. (Alg.) Positive; -- a term applied to quantities which are
      to be added, and opposed to negative, or such as are to
      be subtracted.
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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.
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            Whether there are such beings or not, 't is
            sufficient for my purpose that many have believed
            the affirmative.                      --Dryden.
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   2. A word or phrase expressing affirmation or assent; as,
      yes, that is so, etc.
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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]