noun

• (uncountable) The act of negating something.
• (countable) A denial or contradiction.
• (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".)
• (logic) The logical operation which obtains such (negated) propositions.