Eutychianism was an early heresy which maintained that Jesus Christ was of one nature only.
The heresy was named after Eutyches of Constantinople, who tried in the year 433 to make the 12 Anathemas of Cyril of Alexandria the standard of orthodoxy and do away with the "inspired man" Christology of Antioch. Another goal was to make Alexandria, instead of Constantinople, the second most powerful see in Christendom (next to Rome).
The view of Eutyches was that Christ had only one nature - a confused mixture of human and divine. Eutycianism is also known as monophysitism from monos (single) and physis (nature). It assumes that Christ can have only one nature, which is a mixture of divine nature and human natures, such that the human becomes divine and the divine human. This confuses both Christs true humanity and his true deity. The view was officially deemed heresy by the Council of Chalcedon in 451 CE.