Nontrinitarianism (or antitrinitarianism) refers to monotheistic belief systems, primarily within Christianity, which reject the mainstream Christian doctrine of the Trinity, namely, the teaching that God is three distinct hypostases or persons who are coeternal, coequal, and indivisibly united in one being or ousia.

Nontrinitarianism (or antitrinitarianism) refers to monotheistic belief systems, primarily within Christianity, which reject the mainstream Christian doctrine of the Trinity, namely, the teaching that God is three distinct hypostases or persons who are coeternal, coequal, and indivisibly united in one being or ousia.