necessary

Required, essential, whether logically inescapable or needed in order to achieve a desired result or avoid some penalty.

• Although I wished to think that all was false, it was yet necessary that I, who thus thought, must in some sense exist.
• It is absolutely necessary that you call and confirm your appointment.

Unavoidable, inevitable.

• If it is absolutely necessary to use public computers, you should plan ahead and forward your e-mail to a temporary, disposable account.

Determined, involuntary: acting from compulsion rather than free will.obsolete

• But that a necessary being should give birth to a being with any amount, however limited, of moral freedom, is infinitely less conceivable than that parents of the insect or fish type should give birth to a perfect mammal.

A place to do the "necessary" business of urination and defecation: an outhouse or lavatory.archaiceuphemistic

• It soon became fashionable for even the poorest families to have a necessary not far from their cabin. […] The more affluent settlers painted their necessarys in bright colors and carved their names on the doors.

Necessity.obsolete

• […] loss of that whole dominion of New England, and consequently of their Majesties' other American Plantations, endangered not only by the want of provisions, but by the many ships, vessels, seamen and other necessarys in New England, ...