essential

Etymology

From Late Latin essentiālis, from Latin essentia (“being, essence”).

adj

1. Necessary.
   Thus, research-based resources with the potential to assist teachers prepare secondary students for tertiary education are essential. 2018, Clarence Green, James Lambert, “Advancing disciplinary literacy through English for academic purposes: Discipline-specific wordlists, collocations and word families for eight secondary subjects”, in Journal of English for Academic Purposes, volume 35, →DOI, page 105
2. Very important; of high importance.
   In order to grant the rich these pleasures, the social contract is reconfigured. The welfare state is dismantled. Essential public services are cut so that the rich may pay less tax. […] 2013-05-17, George Monbiot, “Money just makes the rich suffer”, in The Guardian Weekly, volume 188, number 23, page 19
3. (biology) Necessary for survival but not synthesized by the organism, thus needing to be ingested.
4. Being in the basic form; showing its essence.
   Don’t mind him being grumpy. That’s the essential Fred.
5. Really existing; existent.
6. (geometry) Such that each complementary region is irreducible, the boundary of each complementary region is incompressible by disks and monogons in the complementary region, and no leaf is a sphere or a torus bounding a solid torus in the manifold.
7. (medicine) Idiopathic.
   essential blepharospasm
8. Having the nature of essence; not physical.

noun

1. A necessary ingredient.
2. A fundamental ingredient.