any of several doctrines of the Christian church in the 2nd and 3rd centuries a.d., emphasizing the unity of God by maintaining that the Father, the Son, and the Holy Ghost are three manifestations or aspects of God.
Mon·ar·chi·an·ism (mə-när'kē-ə-nĭz'əm) n. Any of several Christian heresies of the second and third centuries A.D. that attempted to maintain monotheism and the unity of the Godhead but thereby denied the independent hypostasis of God the Son.
[From Latin Monarchiānī, the Monarchians, from monarchia, monarchy; see monarchy.] Mo·nar'chi·an n.