What is the meaning of Negation?

The act of negating something.

A denial or contradiction.

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".)

The logical operation which obtains such (negated) propositions.

Source: wiktionary.org