From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
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