Final answer:
During the Great Western Schism from 1378 to 1417, multiple antipopes emerged as contenders for the papacy, leading to divisions within the Catholic Church. The schism ended when the Council of Constance resolved the crisis, uniting the church under Pope Martin V.
Step-by-step explanation:
The antipopes during the Great Western Schism, which lasted from 1378-1417, were individuals who had significant backing and were elected or appointed as popes in opposition to the pope regarded as legitimately elected. The Great Western Schism was a period of division in the Catholic Church where there were rival claimants to the papacy.
After the return of the papal court from Avignon to Rome by Pope Gregory XI in 1377, his successor Pope Urban VI was elected in 1378. However, his election was contested by some cardinals, who elected Clement VII as an antipope in Avignon. This resulted in a split, with some nations recognizing the Roman Pope while others recognized the Avignon Pope.
This schism continued until the election of more antipopes, with Benedict XIII following Clement VII in Avignon, and in opposition, various antipopes in Rome, including Innocent VII and Gregory XII. The schism persisted until the Council of Constance (1414-1418) took measures to end it by persuading two antipopes to resign and formally deposing the third, clearing the way for the election of Pope Martin V as the sole pontiff.