antipope

/ˈantɪpəʊp/

IPA: /AE0 N T IH1 P AH0 P/

noun
  1. 1

    A person who claims or claimed to be the pope, usually as the result of a disputed election or deposition, but is not considered by the Roman Catholic Church to be the real pope.

Source: https://en.wiktionary.org/wiki/antipope