A simple quick reference for Trinitarian words:
ousia (essence/being/substance): Formalized by the council of Nicea (325), it declared the Son to be of the same essence (homoousious) or co-essential with God the Father.
hypostasis (entity, “substance”, or person): Although typically used as a synonym for ousia (the word literally means “to stand under” i.e. “sub-stance”. Theologians will speak of there being one ousia (essence) and three hypostases (entities or “persons”). Why the two terms? Because it was deemed contradictory to say God is one essence and three essences or one substance with three substances. So one synonym was made to serve the other.
prosopon (person, “mask/face”): Literally meaning “face” and implies the persona or image we present to others (commonly used in acting). Although the term is virtually synonymous with hypostasis it did not convey as strong a sense as hypostasis of an actually existing entity/person, as we use the term today and was feared to be a foothold for Modalism. Today speaking of the three “persons” of the Trinity is more accepted and common.
- The classic Trinity formulation of doctrine is that: God is one essence (ousia), existing as three persons (hypostases or prosopon): Father, Son and Spirit.
physis (nature): In reference to the Trinity it is sometimes a synonym for ousia, but in Christology it is a way of speaking of the humanity and divinity of Christ.
hypostatic union: A term used to describe the relationship of the divine and human natures in Christ. In the unity of Christ’s substance/person there is no mixing, confusion or hybrid of the divine and human natures.
- The classic Christology formulation of doctrine is that: Christ is one person (hypostasis or prosopon), who possesses two natures (physes): divine and human.
communicatio idiomatum (the communication or exchange of properties): A Christological precept that, because of Christ’s unity of person, allows for the attributes of the divine and human natures to be described as belonging to the other. We speak of God’s blood or God being born of Mary, dying on the cross….