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 sufficient (def 2).

something necessary or required for a particular purpose; necessity.