The teaching that the Son is the first of all creatures, a god compared to us but not God when compared to God the Father.
This heresy was refuted by the Nicene Creed, composed by the Council of Nicea in 325 AD. The key anti-Arian lines are:
I believe in one Lord Jesus Christ,
the Only Begotten Son of God,
born of the Father before all ages.
God from God, Light from Light,
true God from true God,
begotten, not made, consubstantial with the Father;
through him all things were made.