An antipope is a person who makes a widely accepted claim to be the lawful pope, in opposition to the pope recognized by the Roman Catholic Church. In the past antipopes were typically those supported by a fairly significant faction of cardinals and kingdoms.
Saint Hippolytus (d. 235) is commonly recognized as the earliest antipope, as he protested against Pope Callixtus I and headed a separate group within the Latin Church. Hippolytus was later reconciled to Callixtus's second successor Pope Pontian, when both were condemned to the mines on the island of Sardinia. He has been canonized by the Church. Whether two or more persons have been confused in this account of Hippolytus, and whether Hippolytus actually declared himself to be the Bishop of Rome, remains unclear, especially since no such claim is found in the writings attributed to him.
The period when antipopes were most numerous was during the struggles between the popes and the Holy Roman Emperors of the 11th and 12th centuries. The emperors frequently imposed their own nominees in order to further their cause. The popes, likewise, sometimes sponsored rival imperial claimants in Germany in order to overcome a particular emperor.