An Antipope, is a man (it’s always a man) who claims to be Pope in opposition to the legitimately elected pope.
I’d never heard of an Antipope before picking up the excellent The Middle Ages: A Graphic History by Eleanor Janega. It turns out that there have been many Antipopes – and at times it has been difficult to ascertain which is the ‘real’ Pope.
The Catholic Church, caught in a state of quantum uncertainty, awaiting the theistic-wave-function to collapse and the true vessel of God to reveal himself.