Hypostasis (Greek: ) is the underlying state or underlying substance and is the fundamental reality that supports all else. In Neoplatonism the hypostasis of the soul, the intellect (nous) and "the one" was addressed by Plotinus.
Neoplatonists argue that beneath the surface phenomena that present themselves to our senses are three higher spiritual principles, or hypostases, each one more sublime than the preceding. For Plotinus, these are: the Soul, the Intellect, and the One.
In early Christian writings, hypostasis is used to denote "being" or "substantive reality" and is not always distinguished in meaning from ousia ('essence' or 'substance'). It was used in this way by Tatian and Origen, and also in the anathemas appended to the Nicene Creed of 325.
This section relies largely or entirely on a single source. (July 2015)
It was mainly under the influence of the Cappadocian Fathers that the terminology was clarified and standardized so that the formula "three hypostases in one ousia" came to be accepted as an epitome of the orthodox doctrine of the Trinity. Specifically, Basil of Caesarea argues that the two terms are not synonymous and that they, therefore, are not to be used indiscriminately in referring to the godhead. He writes:
The distinction between ousia and hypostases is the same as that between the general and the particular; as, for instance, between the animal and the particular man. Wherefore, in the case of the Godhead, we confess one essence or substance so as not to give variant definition of existence, but we confess a particular hypostasis, in order that our conception of Father, Son, and Holy Spirit may be without confusion and clear.
This consensus, however, was not achieved without some confusion at first in the minds of Western theologians since in the West the vocabulary was different. Many Latin-speaking theologians understood hypo-stasis as "sub-stantia" (substance); thus when speaking of three "hypostases" in the godhead, they might suspect three "substances" or tritheism. However, from the middle of the fifth century onwards, marked by Council of Chalcedon, the word came to be contrasted with ousia and used to mean "individual reality," especially in the trinitarian and Christological contexts. The Christian concept of the Trinity is often described as being one god existing in three distinct hypostases/personae/persons.