Source : Webster's Revised Unabridged Dictionary (1913)

Negation \Ne*ga"tion\, n. [L. negatio, fr. negare to say no, to
   deny; ne not + the root of aio I say; cf. Gr. ?, Skr. ah to
   say; cf. F. n['e]gation. See {No}, adv., and cf. {Adage},
   {Deny}, {Renegade}.]
   1. The act of denying; assertion of the nonreality or
      untruthfulness of anything; declaration that something is
      not, or has not been, or will not be; denial; -- the
      opposite of {affirmation}.

            Our assertions and negations should be yea and nay.
                                                  --Rogers.

   2. (Logic) Description or definition by denial, exclusion, or
      exception; statement of what a thing is not, or has not,
      from which may be inferred what it is or has.

Source : WordNet®

negation
     n 1: a negative statement; a statement that is a refusal or
          denial of some other statement
     2: the speech act of negating
     3: (logic) a proposition that is true if and only if another
        proposition is false