arianism -
A non-trinitarian Christology, denominated as heretical since Council of Nicaea, but still influential in Monophysite Christianity.
Wikipedia
Arianism is the theological teaching attributed to Arius, a Christian presbyter from Alexandria, Egypt, concerning the relationship of the entities of the Trinity ('God the Father', 'God the Son' and 'God the Holy Spirit') and the precise nature of the Son of God. Deemed a heretic by the First Council of Nicaea of 325, Arius was later exonerated in 335 at the First Synod of Tyre, and then pronounced a heretic again after his death at the First Council of Constantinople of 381. The Roman Emperors Constantius II (337–361) and Valens (364–378) were Arians or Semi-Arians.