adjective
Absolutely necessary or requisite; that one cannot do without.
(of duties, rules etc.) Unbendable, that cannot be set aside or ignored.
(ecclesiastical, obsolete) Not admitting ecclesiastical dispensation; not subject to release or exemption; that cannot be allowed by bending the canonical rules.
noun
A thing that is not dispensable; a necessity.
(in the plural, colloquial, dated) Trousers.