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: Good grooming and hygiene are essential. It's never too early to start being concerned about your appearance - first impressions are everything.
Substantive
Part of speech: adjective
Definition: Of the essence or essential element of a thing; as, "substantive information".Having substance and prompting thought.Applying to essential legal principles and rules of right; as, "substantive law".Of a dye that does not need the use of a mordant to be made fast to that which is being dyed.
Part of speech: noun
Definition: A word that names or refers to a person, place, thing, or idea. Nouns and personal pronouns are always substantives by nature. Other pronouns, numerals and most determiners readily function as substantives in English and many other languages. When present participles function as substantives in English, they are called gerunds; many other languages prefer to use infinitives more often for this purpose. Care in translation is needed when adjectives are used as substantives, because languages differ in which ones are permitted and which ones must remain adjectives.
Example sentence: There's always a great deal of business to be transacted in one's office. There are always visitors it seems to me, an unending stream of them, who come with letters of recommendation, or come actually on substantive business.