Hypostatic union (from the Greek: ὑπόστασις hypóstasis, sediment, foundation, substance, or subsistence) is a technical term in Christian theology employed in mainstream Christology to describe the union of Christ's humanity and divinity in one hypostasis.
Hypostasis had come into use as a technical term prior to the Christological debates of the late fourth and fifth centuries. Before there were Christians, the word was used in Greek philosophy, primarily in Stoicism. Hypostasis had some use in the New Testament that reflect the later, technical understanding of the word; especially Hebrews 1:3. Although it can be rendered literally as "substance" this has been a cause of some confusion so it is now often translated "subsistence". It denotes an actual, concrete existence, in contrast with abstract categories such as Platonic ideals.
Through history 
Apollinaris of Laodicea was the first to use the term hypostasis in trying to understand the Incarnation. Apollinaris described the union of the divine and human in Christ as being of a single nature and having a single essence - a single hypostasis.
The Nestorian Theodore of Mopsuestia went in the other direction, arguing that in Christ there were two natures (dyophysite) (human and divine) and two hypostases (in the sense of "essence" or "person") that co-existed.
The Chalcedonian Definition agreed with Theodore that there were two natures in the Incarnation. However, the Council of Chalcedon also insisted that hypostasis be used as it was in the Trinitarian definition: to indicate the person and not the nature as with Apollinarius.
Thus, the Council declared that in Christ there are two natures; each retaining its own properties, and together united in one subsistence and in one single person (εἰς ἓν πρόσωπον καὶ μίαν ὑπόστασιν, eis hèn prósōpon kaì mían hypóstasin).
As the precise nature of this union is held to defy finite human comprehension, the hypostatic union is also referred to by the alternative term "mystical union."
The Oriental Orthodox Churches, having rejected the Chalcedonian Creed, were known as Monophysites because they would only accept a definition that characterized the incarnate Son as having one nature. The Chalcedonian "in two natures" formula (based, at least partially, on Colossians 2:9) was seen as derived from and akin to a Nestorian Christology. Contrariwise, the Chalcedonians saw the Oriental Orthodox as tending towards Eutychian Monophysitism. However, the Oriental Orthodox have in modern ecumenical dialogue specified that they have never believed in the doctrines of Eutyches, that they have always affirmed that Christ's humanity is consubstantial with our own, and they thus prefer the term Miaphysite to refer to themselves, a reference to Cyrillian Christology, which used the phrase "μία φύσις τοῦ θεοῦ λόγου σεσαρκωμένη", "mía phýsis toû theoû lógou sesarkōménē".
See also 
|Look up hypostasis in Wiktionary, the free dictionary.|
- God's human face: the Christ-icon by Christoph Schoenborn 1994 ISBN 0-89870-514-2 page 154
- Sinai and the Monastery of St. Catherine by John Galey 1986 ISBN 977-424-118-5 page 92
- Systematic Theology by Lewis Sperry Chafer 1993 ISBN 0-8254-2340-6 pages 382-384 
- R. Norris, "Hypostasis," in The Encyclopedia of Early Christianity, ed. E. Ferguson. New York: Garland Publishing, 1997
- Aristotle, "Mund.", IV, 21.
- Placher, William (1983). A History of Christian Theology: An Introduction. Philadelphia: Westminster Press. pp. 78–79. ISBN 0-664-24496-3.
- Gregory of Nyssa, Antirrheticus adversus Apollinarem.
- "Theodore" in The Westminster Dictionary of Christian History, ed. J. Brauer. Philadelphia: Westminster Press, 1971.
- Denzinger, ed. Bannwart, 148