Arianism: A fourth century heresy proposed by a priest named Arius who denied the divinity of Christ. It was condemned by the Council of Nicaea in 325 A.D. The same council defined the term homoousios (one in the same substance or nature) to identify the relationship between Jesus and the Father. Today we say "one in being" in the Nicene Creed. (This Advent we will begin saying "consubstantial.")
This heresy really draws out the history behind a part of the Creed that we say every Sunday. In one sense, we should be thankful for heresy because, ultimately, God uses it to bring forth precious gems like the creed.