WordNet records no synonym and no antonym for negation. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a negative statement; a statement that is a refusal or denial of some other statement
the speech act of negating
Synonyms, antonyms and the broader and narrower relations for negation come from Princeton University’s WordNet. Full sources and licences.
(logic) a proposition that is true if and only if another proposition is false