indispensable

adj

1. 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.”

   “In this paper, I have chosen Khotan, a small oasis-state on the southern edge of the Taklamakan Desert, as the subject for the investigation of this question. The waters of two rivers, the Yurungkash or Baiyu he 白玉河 (White Jade River) and the Karakash or Heiyu he 黑玉河 (Black Jade River), were indispensable for establishing human culture in Khotan.”

   “And my friends in politics, no-one is remotely indispensable and our brilliant and Darwinian system will produce another leader, equally committed to taking this country forward through tough times.”

   “Behind this carefully crafted strategy of wooing Trump is a desire from Gulf states to solidify and formalize their positions as the US’ indispensable security and economic partners, and extract as much benefit for themselves as they can.”

2. 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.”

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

noun

1. A thing that is not dispensable; a necessity.
2. (colloquial, dated, in-plural)Trousers.