antipope

from WordNet (r) 3.0 (2006)

antipope
    n 1: someone who is elected pope in opposition to another person
         who is held to be canonically elected; "the antipopes
         resided in Avignon during the Great Schism"

from The Collaborative International Dictionary of English v.0.48

Antipope \An"ti*pope\, 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]