Homoiousios means "of similar substance," "of the similar essence." It was a term used in the fourth century by a heretical group to describe the relationship between the Father and the Son. This is an improper term to use when describing the relationship between the Father and the Son. They are not of similar essence. Rather, they are of the same essence (Homoousios). In other words, according to the correct doctrine of the Trinity, the Father and the Son (and the Holy Spirit) share the same essence. See Ontological Trinity.