Hypostasis (Greek: ὑπόστασις) 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.
In Christian theology, a hypostasis or person is one of the three persons of the Trinity.
Aristotle used hypostasis in reference to a material substratum underlying change in the unqualified sense of generation and corruption, and otherwise in reference to ousia or substance in a secondary sense for genera and species understood as hylomorphic forms. Primarily, however, he used it with regard to his category of substance, the specimen ("this person" or "this ox") or individual, qua individual, who survives accidental change and in whom the essential properties inhere that define those universals. In contrast, Plato spoke of the objective reality of a thing or its inner reality as opposed to its outer appearance in the Allegory of the Cave.
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 or World-Soul, being/intellect or Divine Mind (Nous), 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.
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 indistinctly 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 view of the Trinity is often described as a view of one God existing in three distinct hypostases/personae/persons.