essential

adj

1. Necessary.

   “There’s canker at the root, your seed Denies the blessing of the sun, The light essential to your need. Your hopes are murdered and undone.”

   “Thus, research-based resources with the potential to assist teachers prepare secondary students for tertiary education are essential.”

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. […]”

3. 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.

   “Or is it true, that thou art but a bare name, / And no eſſential thing?”

6. 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. Idiopathic.

   “essential blepharospasm”

8. Having the nature of essence; not physical.

   “It is usually allowed that there is the natural body, as St. Paul called it, which is dissolved at death, and the etheric or spiritual body which survives and functions upon an etheric plane. Those are the essential things. But we may really have as many coats as an onion and there may be a mental body which may shed itself at any spot where great mental or emotional strain has been experienced.”

noun

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