a 4th-century Trinitarian heresy in the Christian church. Though it modified the extreme position of Arianism, it still fell short of the church's orthodox teaching that Father, Son, and Holy Spirit are of the same substance.

Arius held that the Father, Son, and Holy Spirit were three separate essences (ousiai) or substances (hypostaseis) and that the Son and Spirit derived their divinity from the Father, were created in time, and were inferior to the Godhead. Semi-Arians, however, admitted that the Son was “like” (homoiousios) the Father but not of one substance ( homoousios) with him. This doctrinal controversy, revolving around two words distinguished by a single iota (ι), gave rise to the popular expression, “It makes not one iota of difference.” To Orthodox Christians, however, the iota was of great importance. Both Arianism and semi-Arianism were condemned at the Council of Nicaea (325).