A false claimant of the Holy See in opposition to a pontiff canonically elected. At various times in the history of the Church illegal pretenders to the Papal Chair have arisen, and frequently exercised pontifical functions in defiance of the true occupant. Hergenröther enumerates thirty in the following order:

Hippolytus (?), III century
Novatian, 251
Felix II, 355-365
Ursicinus, 366-367
Eulalius, 418-419
Laurentius, 498-501
Constantine II, 767
Philip, VIII century
Anastasius, 855
Leo VIII, 956-963
Boniface VII, 974
John XVI, X century
Gregory, 1012
Sylvester III, 1044
Benedict X, 1058
Honorius II, 1061-72
Guibert or Clement III, 1080-1100
Theodoric, 1100
Aleric, 1102
Maginulf, 1105
Burdin (Gregory VIII), 1118
Anacletus II, 1130-38
Victor IV, 1159-64
Pascal III, 1164-68
Calixtus III, 1168-77
Innocent III, 1178-80
Nicholas V, 1328-30
Robert of Geneva (Clement VII), 20 September, 1378 to 16 September, 1394
Amadeus of Savoy (Felix V), November, 1439 to April, 1449

Note: See also Dioscorus (d. 530) and John XXIII (1370-1419), not to be confused with the twentieth-century pope of the same name.

Bibliography Information
Obstat, Nihil. Lafort, Remy, Censor. Entry for 'Antipope'. The Catholic Encyclopedia. https://www.studylight.org/encyclopedias/eng/tce/a/antipope.html. Robert Appleton Company. New York. 1914.

Bible Encyclopedias Antipope

The Catholic Encyclopedia

Bible Encyclopedias
Antipope