Homoousian \Ho`mo*ou"si*an\, a. Of or pertaining to the Homoousians, or to the doctrines they held. Homoousian \Ho`mo*ou"si*an\, n. [Gr. ?; ? the same + ? being, essence, substance.] (Eccl. Hist.) One of those, in the 4th century, who accepted the Nicene creed, and maintained that the Son had the same essence or substance with the Father; -- opposed to homoiousian.