EWTN Catholic Q&A
Question from Willy Credo on 04-23-2005:

Catholic Encyclopedia defines that an antipope is a false claimant of the Holy See in opposition to a pontiff canonically elected and Church's illegal pretenders to the Papal Chair. Would you be kind to explain us why some of these antipopes who have been illegal prentenders to the Papal Chair became saints?

Answer by Colin B. Donovan, STL on 04-24-2005:

Antipopes are invalid claimants to the Office of Bishop of Rome. They generally arise when a faction of the cardinals did not like the outcome of an election so they went off and elected their own pope. The man elected in accordance with the norms of law is pope, the other, or others, are antipopes. The last antipope was Felix V, whose claimed reign was 1440-1440.

Sadly, today there are a number of antipopes, deluded individuals who rejecting the popes after Pius XII have gathered a few, and I mean few, followers around themselves and declared, or had themselves declared, pope. May God have mercy on their souls!