An antipope is a person who, in opposition to the one who is generally seen as the legitimately elected Pope, makes a significantly accepted competing claim to be the Pope, the Bishop of Rome and leader of the Roman Catholic Church. At times between the 3rd and mid-15th century, antipopes were supported by a fairly significant faction of religious cardinals and secular kings and kingdoms.
The period in which antipopes were most numerous was during the struggles between the popes and the Holy Roman Emperors of the 11th and 12th centuries. The emperors frequently imposed their own nominees to further their own causes. The popes, likewise, sometimes sponsored rival imperial claimants (anti-kings) in Germany to overcome a particular emperor.
The Western Schism—which began in 1378, when the French cardinals, claiming that the election of Pope Urban VI was invalid, elected Clement VII as Pope—led to two rival lines of claimants to the papacy: the Roman line and the Avignon line (Clement VII took up residence in Avignon, France). To end the schism, with the support of King Sigismund of Germany, a special Catholic council was created to force the competing popes to abdicate, so that another could be chosen.
In May 1415, the Council of Constance forced Pope Gregory XII of the Roman line to resign in July 1415. In 1417, the Council also formally deposed Benedict XIII of Avignon, but he refused to resign. Afterwards, Pope Martin V was elected and was accepted everywhere except in the small and rapidly diminishing area that remained faithful to Benedict XIII.
The scandal of the Western Schism created anti-papal sentiment and fed into the Protestant Reformation at the turn of the 16th century.