indispensable

Absolutely necessary or requisite; that one cannot do without.

An indispensable component of a heart-healthy diet.

But through the oligopoly, charcoal fuel proliferated throughout London's trades and industries. By the 1200s, brewers and bakers, tilemakers, glassblowers, pottery producers, and a range of other craftsmen all became hour-to-hour consumers of charcoal. This only magnified the indispensable nature of the oligopolists.

Unbendable, that cannot be set aside or ignored.

The law was moral and indispensable.

Why is marking a book indispensable to reading it? First, it keeps you awake—not merely conscious, but wide awake.

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

A thing that is not dispensable; a necessity.

Trousers.