An antipope may be defined as
- a pope elected by irregular means or methods
- a pope (or claimant-pope) who holds irregular views
- a pope elected at a time when another pope was already in office.
The main criterion is that of history -- an antipope is the candidate who fails to maintain his claim to the office over the longer term.
Within period, the main antipopes were those of the Great Schism (1378-1415), the period during which the French Kings supported a pope in Avignon, whilst another line of popes sat in Rome. The Schism was ended by the Council of Constance, but not until after a third line of popes had risen, centred on Pisa, in Italy.