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 requisite; 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.

Chiefly New England . a privy or toilet.

adjective

inessential , unimportant , unnecessary , useless , contingent , needless , optional , voluntary , casual , discretional , nonessential

adjective

all-important , basic , binding , bottom-line , cardinal , chief , compelling , compulsory , crucial , decisive , de rigueur * , elementary , exigent , expedient , fundamental , imperative , incumbent on , indispensable , mandatory , momentous , name of game , needed , needful , obligatory , paramount , prerequisite , pressing , prime , principal , quintessential , required , requisite , significant , specified , unavoidable , urgent , vital , wanted , assured , certain , fated , imminent , ineluctable , ineludible , inerrant , inescapable , inevasible , inexorable , infallible , returnless , undeniable , unescapable , alimental , contingent , de rigueur , essential , important , incumbent , indispensableinevitable , inevitable , inherent , integral , intrinsic , perforce , statutory , unpreventable