Patripassianism is a form of modalism, the teaching that there is only one God, who appears in three different modes (as opposed to the orthodox teaching that there is one God, who exists in three persons).
Patripassianism comes from the Latin, and means "the father suffers." The name refers to the teaching that God the Father suffers on the cross as Son — since the two are different modes of the same person. Patripassianism is closely related to Sabellianism.
