Necessary or necessity may refer to:

Need An action somebody may feel they must do An important task or essential thing to do at a particular time or by a particular moment

Necessary and sufficient condition, in logic, something that is a required condition for something else to be the case

Necessary proposition, in logic, a statement about facts that is either unassailably true (tautology) or obviously false (contradiction)

Metaphysical necessity, in philosophy, a truth which is true in all possible worlds

Necessity in modal logic

Law

Other

A bathroom or toilet, in some languages (in English this is an archaic usage)

An economic need enunciated by US President Franklin D. Roosevelt in his 1944 Second Bill of Rights

Necessity (novel), of 2016 by Jo Walton

(novel), of 2016 by Jo Walton Necessary Records, UK record label