Hypostasis (plural: hypostases), from the Greek ὑπόστασις (hypóstasis), 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, the Holy Trinity consists of three hypostases: Hypostasis of the Father, Hypostasis of the Son, and Hypostasis of the Holy Spirit.
Pseudo-Aristotle used hypostasis in the sense of material substance.
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.
Hypostatic union
The term hypostasis has a particular significance in Christian theology, particularly in Christian Triadology (study of the Holy Trinity), and also in Christology (study of Christ).
In Christian Triadology (study of the Holy Trinity) three specific theological concepts have emerged throughout history, in reference to number and mutual relations of divine hypostases:
monohypostatic (or miahypostatic) concept advocates that God has only one hypostasis;
dyohypostatic concept advocates that God has two hypostases (Father and Son);
trihypostatic concept advocates that God has three hypostases (Father, Son and the Holy Spirit). The great majority of Christians, including Catholics, Eastern Orthodox, Oriental Orthodox, and most Protestants, consider trihypostatic Trinitarian doctrine to be definitively Christian, and thus consider other groups which claim to be Christian but do not adhere to this doctrine as "non-Christian".
Within Christology, two specific theological concepts have emerged throughout history, in reference to the Hypostasis of Christ:
monohypostatic concept (in Christology) advocates that Christ has only one hypostasis;
dyohypostatic concept (in Christology) advocates that Christ has two hypostases (divine and human).