The term Great Schism refers to either one of two schisms in the history of Christianity. A schism is a division or split caused by disagreement. Most commonly, "Great Schism" refers to the "great East-West schism", the split between the Eastern and Western churches in the eleventh century; the second schism, the "schism of the west" in the fourteenth century, refers to a time when three (claimant) popes were elected at the same time.
Schism between Western and Eastern Churches
The first schism resulted from among other things: the insertion of the filioque clause into the Nicene Creed; the use by the Western church of unleavened bread for the Eucharist; disputes in the Balkans over whether the Western or Eastern church had jurisdiction. This lead to the exchange of excommunications by the representative of Pope Leo IX and the Patriarch of Constantinople Michael Cerularius in 1054 (finally abolished in 1965) and the separation of the Roman and Orthodox churches. Though there have been frequent attempts to settle differences the breach has not been healed.
Schism inside the Roman Catholic Church
The second, and temporary, schism resulted from the move of the Papacy from Avignon to Rome by Pope Gregory XI in 1378. After Gregory XI died, the Romans rioted to ensure an Italian was elected; the cardinals, fearing the crowds, elected an Italian, Pope Urban VI in 1378; but in the same year the majority of them removed themselves to Fondi, and elected a rival Pope from there, Pope Clement VII. Later a council at Pisa was held in 1409 to try to solve the dispute, but it only resulted in the election of a third Pope, Pope Alexander V by the council, soon to be followed by Pope John XXIII. Finally, the Council of Constance in 1417 deposed John XXIII and the Avignon Pope Benedict XIII, received the resignation of the Roman Pope Gregory XII, and elected Pope Martin V, thereby ending the schism. The alternate papal claimants have become known in history as antipopes.