indispensable

Short definition from Wiktionary

Adjective

1. 1
   Not admitting ecclesiastical dispensation; not subject to release or exemption; that cannot be allowed by bending the canonical rules.
2. 2
   Unbendable, that cannot be set aside or ignored.
3. 3
   Absolutely necessary or requisite; that one cannot do without.

Noun

1. 1
   A thing that is not dispensable; a necessity.
2. 2
   Trousers.