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 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.