homoousios''' is a Greek word meaning " one substance" or " one essence." It is used in the [[Nicene Creed]] to say that Jesus Christ is of one essence with the Father. Although it does not appear in the Bible, the fathers of the [[First Ecumenical Council]] ultimately decided that it this was the best language to use concerning the [[Holy Trinity]] .
The competing term at that council was ''homoiousios'' meaning essences rather than one essence; it was favored by the [[Arianism|Arians]]. Because of how close these two words are in the Greek, it has been said that there was only "one iota" of difference between them.