Confusion of terms in Patristic theology

Much of the controversies that raged the Patristic period regarding Trinity and Christology were due to the confusion of meanings in the usage of Greek terms in the Alexandrian and Antiochean Schools. Dr. V. C. Samuel in his Council of Chalcedon Re-Examined (1977) writes: "In the Christological controversy, unlike any other theological dispute in the ancient church, there was a great deal of obscurity on account of the technical terms that were employed. theologians of all shades of opinion used the following terms: ousia in Greek; the Syriac used either ousia or Ithutho to correspond to it. Hupostasis in Greek and its Syriac equivalent Qnumo. Physis in Greek had kyono as its Syriac rendering. The Greek prosopon had parsupo in Syriac....(p.218,;see also , pp. 257-9).

For a brief description of this confusion of terms visit Timothy Enloe's site.

He writes:

"Prior to the Councils, at least in Neoplatonic circles, the terms hypostasis (υποστασισ) and ousia (ουσια) had the same meaning. Whereas hypostasis signified a real being that had entered real existence, ousia meant something primal and eternal. Significantly for later disputes, Latin translations of the creeds used the word substantia for both hypostasis and ousia.

The word prosopon (πρωσοπον) classically referred to one’s “face” in the sense of “individual outward being.” Its meaning could be extended to refer to one’s “role” or “position” in the community. In patristic thought, as a result of the Sabellian controversies, prosopon came to mean roughly the same thing as hypostasis (but with no metaphysical connotations) and sometimes also referred to a concrete instantation of an abstract ousia.

Lastly, the word physis (φυσισ) typically referred to the actual nature of a thing, to the characteristics and qualities of which it was composed or to those things as they would exist when it was finished developing.

At the Council of Nicea (325 A.D.), hypostasis and ousia were used interchangeably against the Arian doctrine that “there was a time when He was not.” It is possible that both terms were used to indicate some differences of emphasis among the assembled Fathers. By contrast, by the time of the Council of Constantinople (381 A.D.), hypostasis had become synonymous instead with prosopon. The common triune nature of the Godhead became ousia while the individual expression of the three Persons were each prosopon. Meanwhile, physis became tied to ousia in another refutation of the Arian position that Christ was a creation of God (i.e., a physis with a different ousia)."

Also go to the descriptions of Key Terms

ousia (essence/being/substance): Important in the Trinitarian doctrine codified at the council of Nicea (325), which declared the Son to be of the same essence (homoousious) as the Father. It becomes important in Christological debates as various theologians sought to understand how the humanity of Jesus and our human nature could be homoousious (usually translated "co-essential").

hypostasis (entity, substance, "person"): Used at Nicea as a synonym for ousia (the word literally means "to stand under" i.e. "sub-stance"). Over the course of the 4th century it comes to be distinguished from ousia and taken as meaning "entity" or "individual reality" or even "distinct manner of existing." Thus in speaking of the Trinity, theologians will speak of there being one ousia (essence/being/substance) and three hypostases (entities or "persons"). In Christology there is a concern that while Christ is both fully human and fully divine, this duality of nature not be thought of as compromising the unity of Jesus Christ as a single hypostasis i.e. Jesus is only one "thing."

physis (nature): Sometimes used in Trinitarian theology as a synonym for ousia, the term really comes into its own in Christological reflection as a way of speaking of the humanity and divinity of Christ. Thus the unity of Christ's hypostasis (person) does not imply any mixing of the divine nature and the human nature.

prosopon (person, "mask/face"): This term literally means "face" and implies the "person" that we present to others. This term was used in both Trinitarian and Christological reflection as virtually synonymous with hypostasis. However, it did not convey as strong a sense as hypostasis of an actually existing entity and thus was suspected by some of indicating only the appearance of a distinct and unified act of existence.
In Trinitarian theology, the "classic" formulation of doctrine is that
God is one essence (ousia)
existing as three persons (hypostases): Father, Son and Spirit.

In Christology, the "classic" formulation of doctrine is that
Christ is one person (hypostasis)
who possesses two natures (physes): divine and human.