Heteroousianism is a Christian belief that the substance or being of God the Father

God the Father

God the Father is a gendered title given to God in many monotheistic religions, particularly patriarchal, Abrahamic ones. In Judaism, God is called Father because he is the creator, life-giver, law-giver, and protector...

 and the substance or being of the Son of God

Son of God

"Son of God" is a phrase which according to most Christian denominations, Trinitarian in belief, refers to the relationship between Jesus and God, specifically as "God the Son"...

 (Jesus

Jesus

Jesus of Nazareth , commonly referred to as Jesus Christ or simply as Jesus or Christ, is the central figure of Christianity...

) are different. Commonly called Arianism

Arianism

Arianism is the theological teaching attributed to Arius , a Christian presbyter from Alexandria, Egypt, concerning the relationship of the entities of the Trinity and the precise nature of the Son of God as being a subordinate entity to God the Father...

, though Arianism includes other beliefs in addition to this one. The teaching was developed as a response to the homoousian

Homoousian

Homoousian is a technical theological term used in discussion of the Christian understanding of God as Trinity. The Nicene Creed describes Jesus as being homooúsios with God the Father — that is, they are of the "same substance" and are equally God...

 teaching.

The term derives from the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 , heterooúsios, "differing in substance" from , héteros, "another" and , 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.