An antipope is one whose claim to being Pope is the result of a disputed or contested election. These antipopes were usually in opposition to a specific person chosen by the papal electors (since the Middle Ages, the college of cardinals). Some self-appointed leaders of smaller churches are also called "antipopes."
During certain periods of turbulence in the Roman Catholic Church, controversial Papal elections were conducted. Some such elections were considered invalid, either because a large majority of papal electors claimed the election was invalid (such as the election of Felix V), or because they have subsequently been declared invalid (such as Clement VII).
The earliest antipope, Hippolytus, was elected in protest against Pope Callixtus I by a schismatic group in the city of Rome in the 3rd century. Hippolytus was exiled to the mines on the island of Sardinia in the company of Callixtus' successor Pope Pontian, and was reconciled to the Catholic Church before his death.
The late 14th and early 15th century saw a series of rival popes elected, one line of which is counted by the Roman Catholic Church as popes and the other as antipopes. The scandal of multiple claimants added to the demands for reform that produced the Protestant Reformation at the turn of the 16th century. (See Western Schism, Antipope Benedict XIII.)
It would not necessarily have been evident, during periods when two (or three) rival claimants existed, which was the antipope, and which was the pope, and the clear-cut distinctions made between them in retrospect can give a false sense that certainty existed among their contemporaries. Supporters might offer assistance to a given candidate, but could not know which would be determined to have been an antipope, and which the pope, until events had run their course.
There has not been an antipope since 1449, unless sedevacantist antipopes are counted (see below). Other schisms like the Church of England are controlled by lay sovereigns who do not want to have an ecclesiastical rival or begin like the Old Catholic Church and the Chinese Patriotic Catholic Association in a rejection of a primary dogma of the papacy.
Today the act of becoming an Antipope is considered a schismatic act by the Roman Catholic Church. This would result in automatic excommunication for the person who became Antipope.
List of antipopes
St. Hippolytus(reconciled with Pope St. Pontian and died as martyr to the church),217–235
Some breakaway Catholics today, called sedevacantists, claim the current Popes are heretics for replacing the Tridentine Latin Mass with the Novus Ordo Missae; many of them also object to the celebration of the Mass in the vernacular. Since the opinion of many Catholic theologians is that a heretical Pope would cease to be Catholic and therefore cease to be Pope, sedevacantists believe the current Bishops of Rome are not actually popes. Some sedevacantist groups have their own popes to replace the popes they reject. They are sometimes called antipopes, although it should be noted that in contrast to historical antipopes, the number of their followers is minuscule.