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.