—adj. 1. being essential, indispensable, or requisite: a necessary part of the motor. 2. happening or existing by necessity: a necessary change in our plans. 3. acting or proceeding from compulsion or necessity; not free; involuntary: a necessary agent. 4. Logic. a. (of a proposition) such that a denial of it involves a self-contradiction.
b. (of an inference or argument) such that its conclusion cannot be false if its supporting premises are true.
c. (of a condition) such that it must exist if a given event is to occur or a given thing is to exist. Cf. sufficient (def. 2).
—n. 1. something necessary or requisite; necessity.
2. necessaries,Law.food, clothing, etc., required by a dependent or incompetent and varying with his or her social or economic position or that of the person upon whom he or she is dependent.
3. Chiefly New Eng.a privy or toilet.