(n) someone who is elected pope in opposition to another person who is held to be canonically elected, Example:the antipopes resided in Avignon during the Great Schism
The Collaborative International Dictionary of English (GCIDE) v.0.53 Collaborative International Dictionary (GCIDE)
n. One who is elected, or claims to be, pope in opposition to the pope canonically chosen; esp. applied to those popes who resided at Avignon during the Great Schism. [ 1913 Webster ]