That which establishes a thing’s being, that by which a thing is what it is (id quo res est, id quod est).
In Christianity, the Word of God (“by whom all things were made”) is the locus of essences.
More specifically
Essence designates what constitutes the proper nature of a being, what makes it intelligible even before its actual existence. It expresses the quidditas — the “whatness” — in distinction from existence, the “that it is.” For Aristotle, essence is the intelligible form of substance, that which remains identical through the variety of accidents. For Thomas Aquinas, essence and existence are distinct in created beings, yet coincide in God, whose essence is existence itself (ipsum esse subsistens).
In the Christian metaphysical view, essences are precontained in the Divine Word, the source and model of all creation: “through Him all things were made” (John 1:3). Each essence thus participates in the First Being while manifesting, at its own level, an aspect of the Principle. The “essential” is not only what is fundamental, but what refers back to the Origin, to the pure intelligibility of being.
Metaphysical knowledge does not stop at essences: it passes through them toward their source — Being itself. Where the intellect apprehends forms, intellectual intuition, or the “intelligence of principles,” contemplates their ground.
For further reading
René Guénon, The Multiple States of the Being, Gallimard, 1932.
Jean Borella, The Crisis of Religious Symbolism, L’Harmattan, 1984.
Bruno Bérard, Metaphysics for Everyone (Angelico Press), trad. of Métaphysique pour tous (Paris, L’Harmattan, 2021); Ital. transl. Sui sentieri della metafisica; Span. ¿Qué es la metafísica?; Ger. Was ist Metaphysik? – on the distinction between essence and existence within the hierarchy of being and participation in the Divine Being.