antipope
noun
an·ti·pope ˈan-ti-ˌpōp 
: one elected or claiming to be pope in opposition to the pope canonically chosen
Example Sentences
Recent Examples on the Web The oldest item in the show is a 12th-century parchment papal bull, signed by the reigning pope, Anastasius IV; three future popes; one future antipope; and one future saint. 
Jennifer Schuessler, New York Times, 12 July 2018
Jennifer Schuessler, New York Times, 12 July 2018 Word History
Etymology
Middle English antepope, from Middle French antipape, from Medieval Latin antipapa, from anti- + papa pope
First Known Use
15th century, in the meaning defined above