Essential
Part of speech: adjective
Definition: Necessary. Very important; of high importance. Being in the basic form; showing its essence. 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.
Part of speech: noun
Definition: A necessary ingredient. A fundamental ingredient.
Example sentence: Water and air, the two essential fluids on which all life depends, have become global garbage cans.
Necessity
Part of speech: noun
Definition: The quality or state of being necessary, unavoidable, or absolutely requisite; inevitableness; indispensableness.The condition of being needy or necessitous; pressing need; indigence; want.That which is necessary; a necessary; a requisite; something indispensable; -- often in the plural.That which makes an act or an event unavoidable; irresistible force; overruling power; compulsion, physical or moral; fate; fatality.The negation of freedom in voluntary action; the subjection of all phenomena, whether material or spiritual, to inevitable causation; necessitarianism.
Example sentence: Classic art was the art of necessity: modern romantic art bears the stamp of caprice and chance.