They accept that God is One but deny that He is Three.
They teach that Father, Son and Holy Spirit are simply three ROLES (or modes) by which the One God reveals Himself
ex: one man may be a son to his father, a husband to his wife, and a father to his children
This teaching is held by the United Pentecostals, Apostolic Church, and others who deny that God is 3 in any sense whatsoever
Oneness, Jesus only or Jesus Name Movement, Modalism