necessary

Needed or required: synonym: indispensable.

Unavoidably determined by prior conditions or circumstances; inevitable.

Logically inevitable.

Required by obligation, compulsion, or convention.

Something indispensable.

Such as must be; that cannot be otherwise.

Such that it cannot be disregarded or omitted; indispensable; requisite; essential; needful; required: as, air is necessary to support animal life; food is necessary to nourish the body.

In law:

Requisite for reasonable convenience and facility or completeness in accomplishing the purpose intended: as, the land necessary for building a railroad.