indispensable

A thing that is not dispensable; a necessity.

(in the plural) Trousers.

Not admitting ecclesiastical dispensation; not subject to release or exemption; that cannot be allowed by bending the canonical rules.

(of duties, rules etc.) Unbendable, that cannot be set aside or ignored.

“The law was moral and indispensable. -Bp. Burnet”

Absolutely necessary or requisite; that one cannot do without.

“An indispensable component of a heart-healthy diet.”