necessary

necessary/ ˈnɛs əˌsɛr i /

adjective
being essential, indispensable, or requisite:
a necessary part of the motor.
happening or existing by necessity:
a necessary change in our plans.
acting or proceeding from compulsion or necessity; not free; involuntary:
a necessary agent.
Logic.
(of a proposition) such that a denial of it involves a self-contradiction.
(of an inference or argument) such that its conclusion cannot be false if its supporting premises are true.
(of a condition) such that it must exist if a given event is to occur or a given thing is to exist.Compare sufficien.

noun, plural nec·es·sar·ies.
something necessary or required for a particular purpose; necessity.
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.

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