The Arian controversy was a huge debate in the early church in the early 300s concerning the nature of Jesus.
Arius, a church leader from Alexandria, had a view of Christ that stated that Jesus was different in substance from the Father and the Holy Spirit. This idea was rooted in passages of the gospel of John. For example, in John 14:28, Jesus states that his father is greater than he is. Arius argued in view of verses like this that the son has a similar substance (homoiousios) to the Father and therefore denied a more traditional understanding of the trinity. More specifically, he argued that the Son was like the Father, but derivative. The Greek word he used to describe the nature of Christ was "homoiousios."
To us, this might seem like a minor point, but the early church debated this theological point carefully. They even held a council under the emperor Constantine in 325 A.D. It was called the council of Nicaea. In this council, the bishops and leader concluded that Jesus was of the same substance as the father. They rejected the idea of "homoiousios" and stated that Jesus was of the same substance "homoousios." The "i" made all the difference.
Also Arius was condemned as a heretic. He was subsequently acquitted for a time being. But at the council of Constantinople (381), he was rejected once again and this time for good.