negation

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act or process of negating.
• noun A denial, contradiction, or negative statement.
• noun The opposite or absence of something regarded as actual, positive, or affirmative.

from The Century Dictionary.

• noun The act of denying or of negativing; the opposite of the act of affirming.
• noun A denial; a declaration that something is not, or has not been, or will not be.
• noun The absence of that which is positive or affirmative; blankness; emptiness.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun uncountable The act of negating something.
• noun countable A denial or contradiction.
• noun logic, countable A proposition which is the contradictory of another proposition and which can be obtained from that other proposition by the appropriately placed addition/insertion of the word "not". (Or, in symbolic logic, by prepending that proposition with the symbol for the logical operator "not".)
• noun logic The logical operation which obtains such (negated) propositions.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a negative statement; a statement that is a refusal or denial of some other statement
• noun (logic) a proposition that is true if and only if another proposition is false
• noun the speech act of negating