The Collaborative International Dictionary
Semi-Arianism \Sem`i-A"ri*an*ism\, n. The doctrines or tenets of the Semi-Arians.
Wikipedia
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.