The teaching that the Son of God was a creature "of like substance" (homoiousios), though not identical with God. It is named for Arius, a fourth-century presbyter of Alexandria who made a highly influential (if not especially original) contribution to the discussion of the proper way to express the relationship between God and the Son within the divine life. Arius was not primarily interested in the relationship between the divinity and humanity of Jesus. He was concerned to preserve the unity, sole eternity, and self-existence of God. The famous epitome of Arius's position is, "There was when the Son was not."
Glossary definitions provided courtesy of Church Publishing Incorporated, New York, NY,(All Rights reserved) from “An Episcopal Dictionary of the Church, A User Friendly Reference for Episcopalians,” Don S. Armentrout and Robert Boak Slocum, editors.