Hypostatic

Hypostatic union is a theological term that expresses that Christ is one person (prosopon) who subsists in two natures human and divine; this is therefore related to the doctrine of the Incarnation. The term "hypostasis" means literally "that which lies beneath," and is also referred to as the mystical union. More simply, the doctrine states that Jesus is both fully human and fully divine. Included in this is the doctrine of Dythelitism, i.e., that Christ has two wills, which always act in union. These doctrines were pronounced by the Ecumenical Councils of Ephesus, Chalcedon, and Constantinople.
The term "hypostasis" was used by some Greek philosophers to distinguish reality from appearances, and, before its theological employment by the Council of Nicaea, it was synonymous with "substance" or "being" The subtle theological distinction was fully expressed by the Council of Chalcedon, which declared that the one substance and one person of Christ was in two natures, each perfectly united yet with each retaining its own properties .
Groups that reject either the divinity or humanity of Jesus obviously do not hold the doctrine of hypostatic union. However, some groups hold that Jesus is both man and God, but employ different teachings to explain this relationship. Nestorianism holds that Christ not only has two natures, but that he is two physical persons united morally, but not physically, by means of grace. Monophysitism holds that Jesus has only one nature: either his human nature is wholly absorbed by the divine, or the converse, or that the two are mixed such that a third nature results, which supersedes its constituent human and divine components. Monothelitism holds that, though Christ has two natures, he only has one will. Many of these views found renewed forms in Western Christianity at the time of the Reformation, especially among Adoptionists, Socinians, and Ubiquitarians.