The Western Schism or Papal Schism was a split within the Catholic Church in 1378. Lacking any real theological or doctrinal underpinnings, being rather driven by politics, it was resolved after 40 years by the Council of Constance. It is occasionally called the Great Schism, though this term is more often applied to the East-West Schism of 1054.
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. Urban had been a respected administrator in the papal chancery at Avignon; but once he was elected Pope, he became suspicious, overbearing, and subject to violent outbursts of temper. The cardinals who had elected him soon came to repent of their decision, and on September 20 the same year, the majority of them removed themselves to Fondi, and elected a rival Pope from there, who took the title of Pope Clement VII.
The two popes threw the Church into a turmoil; there had been antipopes, rival claimants to the papacy before, but most of them had been appointed by various rival factions. Here, the acknowledged and legitimate leaders of the Church themselves had created the two rival popes. European secular leaders had to choose which pope they would recognize; generally, France, Aragon, Burgundy, Savoy, Naples, and Scotland chose to recognise the Avignon claimants, while England, Germany, northern Italy and central Europe of the Holy Roman Empire followed the Roman claimant. Even saints were caught up in the dispute; St Catherine of Siena defended Urban's papacy, while St Vincent Ferrer was in Clement's camp.
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.
Thus the line of Roman popes was recognized as the legitimate line. Consistent with this outcome, from this time forward in the Catholic church it was decreed explicitly that no Council had power over the Popes, and there is no way to undo a Papal election by anyone but the pope.
The alternate papal claimants have become known in history as antipopes.
- Catholic Encyclopedia article