Antipope

An antipope (Latin: antipapa) or anti-pope 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 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. Persons who claim to be pope, but have few followers, such as the modern sedevacantist antipopes, are not classified with the historical antipopes.

In the list of popes given in the Holy See's annual directory, Annuario Pontificio, the following note is attached to the name of Pope Leo VIII (963–965):

At this point, as again in the mid-eleventh century, we come across elections in which problems of harmonising historical criteria and those of theology and canon law make it impossible to decide clearly which side possessed the legitimacy whose factual existence guarantees the unbroken lawful succession of the successors of Saint Peter. The uncertainty that in some cases results has made it advisable to abandon the assignation of successive numbers in the list of the popes.