NOUN (3)

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;

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            Our assertions and negations should be yea and nay.
                                                  --Rogers.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

141 Moby Thesaurus words for "negation":
   abnegation, abolishment, abolition, absence, annihilation,
   annulment, antagonism, argumentation, bucking, challenge, choking,
   choking off, clashing, conflict, contention, contradiction,
   contraposition, contravention, contraversion, controversy,
   counteraction, counterworking, crosscurrent, crossing, declension,
   declination, declinature, declining, denial, deprivation,
   deracination, difference, disaccord, disaccordance, disagreement,
   disallowance, disclaimer, disclamation, disconfirmation, discord,
   discordance, discordancy, discrepancy, disharmony, disobedience,
   disparity, disproof, disproval, disproving, dissension, dissent,
   dissidence, dissonance, disunion, disunity, divergence, diversity,
   elimination, emptiness, eradication, explosion, expose, exposure,
   extermination, extinction, extinguishment, extirpation, faction,
   gainsaying, head wind, holding back, impugnation, impugnment,
   inaccordance, inequality, invalidation, jarring, liquidation, nay,
   negative, negative answer, negativeness, negativity, nihility, nix,
   no, nonacceptance, nonbeing, noncompliance, nonconsent, nonentity,
   nonexistence, nonobservance, nonoccurrence, nonreality,
   nonsubsistence, not-being, nothingness, nullification, nullity,
   opposing, opposition, opposure, oppugnancy, oppugnation, purge,
   rebutment, rebuttal, recantation, redargution,
   reductio ad absurdum, refusal, rejection, repudiation, repugnance,
   resistance, retention, rooting out, silencing, snuffing out,
   standing against, stifling, strangulation, suffocation,
   suppression, thumbs-down, traversal, turndown, unactuality,
   undercurrent, unharmoniousness, unreality, unwillingness,
   uprooting, vacancy, vacuity, vacuum, variance, void, voiding,
   withholding



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

NEGATION. Denial. Two negations are construed to mean one affirmation. Dig.
50, 16, 137.