Ousia

Ousia is a Greek term meaning being, essence, or substance. In philosophy, it denotes the fundamental nature of a thing—the characteristic that makes a thing what it is, as opposed to its accidents or contingent properties. The word is most closely associated with Aristotle, who treated substances as the primary beings and used ousia to denote the kind of reality that can be predicated of a subject. In his taxonomy, primary substances (individuals) and, more broadly, the kinds or species referred to by secondary substances are organized around a common underlying ousia.

In Christian theology, ousia takes on a technical use related to the nature of God. The term is paired with hypostasis (often translated as “person” or “subsisting reality”) to articulate the Trinity. The goal was to affirm the unity of God’s essence while acknowledging the distinct realities of Father, Son, and Spirit. At the Council of Nicaea in 325, the creed affirms that the Son is of the same ousia as the Father, using the term homoousios (“of the same essence”). Later, Eastern and Western theologians often expressed the formula as one ousia in three hypostases, or one essence and three persons, to maintain both unity and distinction within the Godhead.

Across philosophy and theology, ousia thus functions as a foundational term for discussions of what makes something the kind of being it is, and, in Trinitarian doctrine, for describing the shared divine nature of the Trinity. The term continues to be used in scholarly and theological discussions, typically translated as essence or substance depending on the context.