Heteroousianism (or heterousianism) is a Christian belief that the substance or being of God the Father and the substance or being of the Son of God (Jesus) are different. Commonly called Arianism, though Arianism includes other beliefs in addition to this one. The teaching was developed as a response to the homoousian teaching.
The term derives from the Greek ἑτεροούσιος, heterooúsios, "differing in substance" from ἕτερος, héteros, "another" and οὐσία, ousía, "substance, being".
The hardline form of heteroousianism, in which the substance of the Father and Son are not only said to be different, but also dissimilar, is usually called Anomoeanism.