Negation

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.

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.

(logic) a proposition that is true if and only if another proposition is false