The Great Schism of 1054 caused a split between the See of Rome (now the Roman Catholic Church) and the other Christian Patriarchates. This division is the subject of many talks between Western and Eastern Christians.
The Name of the Event
Historical recounts—whether from the Church or secular—have used the year 1054 as the point where the See of Rome split from the Church. However, it is misleading because it implies that before 1054 was alright and after 1054 there was animosity. The events that led up to the split took several centuries to crystalize into a split. Likewise, the split itself took a while after 1054. (Some people would mark 1204, the year of the Fourth Crusade, as the straw that broke the camel's back as far as Rome-Church relations.)
"The Great Ecumenical Schism" is the preferred term to succinctly explain what happened and to capture the complexity of the event itself. This is especially the case for discussing this in a Western audience because the name "The Great Schism" refers also to what happened in the 14th century involving the location of the Pope being either in Rome or in Avignon. This event is also called the "Babylonian Captivity."
The Dogmatic Matters: The Filioque
While there were many other factors at work in the split, the central idea that caused a separation in the place was dogmatic. As soon as the See of Rome endorsed the idea of the Filioque, there is a split between the true faith and a schismatic faith. Also, as long as the See of Rome continues to make it official dogma, there is still a schism.
To summarize an already extensive article on the matter, the Filioque is a word that changes the Nicene Creed into "[Spiritus Sanctus] ex Patre Filioque procedit" or "[Holy Spirit] proceeds from the Father and the Son." The first appearance into the Creed happened in Spain when Latin theologians were trying to refute a brand of the Arian heresy. The theologians had better access to the writings of Latin theologians, particularly of St. Augustine of Hippo. Augustine had the notion that the Holy Spirit proceeded from the Father and the Son but that neither were subordinate to each other. So the Creed was changed by a local synod of bishops and the justification was that it both asserts the divinity of Christ (refuting Arianism) and the unity of the Trinity.