Filioque is a Latin word meaning "and the Son" which was added to the Nicene-Constantinopolitan Creed by the Church of Rome in the 11th century, one of the major factors leading to the Great Schism between East and West. This inclusion in the Creedal article regarding the Holy Spirit thus states that the Spirit "proceeds from the Father and the Son."
Its inclusion in the Creed is a violation of the canons of the Third Ecumenical Council in 431, which forbade and anathematized any additions to the Creed, a prohibition which was reiterated at the Eighth Ecumenical Council in 879-880. This word was not included by the Council of Nicea nor of Constantinople, and most in the Orthodox Church consider this inclusion to be a heresy.
The description of the filioque as a heresy was iterated most clearly and definitively by the great Father and Pillar of the Church, St. Photius the Great, in his On the Mystagogy of the Holy Spirit. He describes it as a heresy of Triadology, striking at the very heart of what the Church believes about God.