The concept of hypostasis as the shared existence of spiritual and corporal entities has been used in a number of religious and intellectual settings. The word hypostasis (Greek ὑπόστασις) means underlying state or underlying substance, and is the fundamental reality that supports all else.
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, being/intellect (Nous), and the One.
In Christian usage, the Greek word hypostasis (ὑπόστᾰσις) means beneath-standing or underpinning and, by extension, the existence of some thing. It can also mean manifestation.
In Early Christian writings it 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. See also: Hypostatic union, where the term is used to describe the union of Christ's humanity and divinity. The term has also been used and is still used in modern Greek (not just Koine Greek or common ancient Greek) to mean "existence" along with the Greek word hýparxis (ὕπαρξις) and tropos hypárxeos (τρόπος ὑπάρξεως), which is individual existence.
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 Holy 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. This led 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.