A heretical system, begun about the 2nd century, which so distorts the monarchy (monarchia) of God as to deny the distinction of Persons in the Trinity.
It takes the three following forms:
(1) Crass Monarchianism absolutely denies any distinction of Persons in the Trinity and concludes naturally that the Father became incarnate and died on the cross.
Hence the name of Patripassians.
(2) Modalist Monarchianism, championed by Sabellius c.50, recognized a distinction in the Trinity, not of Persons but of energies or modes.
The Deity is one Person but manifests Itself under different modes; in creation as the Father, in redemption as the Son, in sanctification as the Holy Ghost.
Sabellius was condemned by Pope Callistus.
(3) Subordinationism, which admits a real and true distinction of Persons in the Trinity but denies equality between them.
New Catholic Dictionary