Creed of Sirmium

Semi-Arianism was a position regarding the relationship between God the Father and the Son of God, adopted by some 4th century Christians. Though the doctrine modified the teachings of Arianism, it still rejected the doctrine that Father, Son, and Holy Spirit are of the same substance, or con-substantial, and was therefore considered to be heretical by many contemporary Christians.Semi-Arianism is a name frequently given to the Trinitarian position of the conservative majority of the Eastern Christian Church in the 4th century, to distinguish it from strict Arianism.

Arius held that the Father, Son, and Holy Spirit were three separate essences or substances (ousiai or hypostases) and that the Son and Spirit derived their divinity from the Father, were created, and were inferior to the Godhead of the Father. Semi-Arians, however, admitted that the Son was “of a similar substance” (homoiousios) as the Father but not "of the same substance" (homoousios) as him. This doctrinal controversy revolved around two words that in writing differed only by a single letter but whose difference in meaning gave rise to furious contests.

Arianism was the view of Arius and his followers, the Arians, that Jesus was subordinate to, and of a different being (ousia) to God the Father. Arians opposed the catholic view that the three persons of the Trinity were of one being or substance. Arianism spread among the Church of Alexandria and the Eastern Mediterranean. After the First Council of Nicaea condemned Arianism as heresy, many Christians adopted compromise views in which they remained in communion with Arians without adopting Arianism itself. Various formulae, such as the homoiousian and the homoean, were proposed to compromise between Arian teachings (heteroousios) and the doctrine of one substance (homoousios) asserted in the Nicene Creed.

...
Wikipedia