Hypostasis (plural: hypostases), from the Greek ὑπόστασις (hypóstasis), is the underlying, fundamental state or substance that supports all of reality. It is not the same as the concept of a substance.^{[citation needed]} 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: that of the Father, that of the Son, and that 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.

The term hypostasis has particular significance in Christian theology; particularly in Christian triadology—the study of the Christian doctrine of the trinity—as well as Christology (study of Jesus as Christ).

In Christian triadology, three specific theological concepts have emerged throughout history in reference to the number and interrelationship of the hypostases:

The monohypostatic concept advocates that the Father, Son, and Spirit are one single hypostasis in a single ousia—meaning that the Father, Son, and Spirit are a single Person. Historically, there were variations of this view:

Among the pre-Nicene Church Fathers, "Dionysius of Rome ... said that it is wrong to divide the divine monarchy 'into three ... separated hypostases ... people who hold this in effect produce three gods'."

In the fourth century, Sabellians (such as Eustathius and Marcellus,), Alexander, Athanasius, and the Western Church taught a single hypostasis in God. The "clear inference from [Athanasius'] usage" is that "there is only one hypostasis in God." Some leading scholars claim that even the Nicene Creed professes a 'one hypostasis' theology.