Miaphysitism is the Christological doctrine that holds Jesus, the "Incarnate Word, is fully divine and fully human, in one 'nature' (physis)." It is a position held by the Oriental Orthodox Churches and differs from the Chalcedonian position that Jesus is one "person" (ὑπόστασις) in two "natures" (φύσεις), a divine nature and a human nature (Dyophysitism).
While historically a major point of controversy within Christianity, several modern declarations by both Chalcedonian and Miaphysite churches state that the difference between the two Christological formulations does not reflect any significant difference in belief about the nature of Christ.
The word miaphysite derives from the Ancient Greek μία (mía, "one") plus φύσις (phúsis, "nature, substance"). Miaphysite teaching is based on Cyril of Alexandria's formula μία φύσις τοῦ θεοῦ λόγου σεσαρκωμένη, meaning "one physis of the Word of God made flesh" (or "... of God the Word made flesh").
The 451 Council of Chalcedon used physis to mean "nature" (as in "divine nature" and "human nature"), and defined that there was in Jesus one hypostasis (person) but two physeis (natures). It is disputed whether Cyril used physis in that sense. John Anthony McGuckin says that in Cyril's formula "physis serves as a rough semantic equivalent to hypostasis".
Others interpret the Miaphysite term physis in line with its use by the Council of Chalcedon and speak of "Miaphysitism" as "Monophysitism", a word used of all forms of denial of the Chalcedonian doctrine. However, they add that "Miaphysitism" is "the more accurate term for the position held by the Syriac, Coptic and Armenian churches".
The Second Council of Constantinople (553), the ecumenical council that followed that of Chalcedon, accepted Cyril's phrase, but warned against misinterpreting it.
The broad term "Dyophysitism" covers not only the Chalcedonian teaching but also what Nestorianism interpreted as meaning that Jesus is not only of two natures but is in fact two centres of attribution, and thus two persons, a view condemned by the Council of Chalcedon.