Since I've never heard the word before.
Sedevacantism - a doctrinal position within traditionalist Catholicism which holds that the present occupier of the Holy See is not a valid pope due to the mainstream Catholic church's alleged espousal of modernism and that, for lack of a valid pope, the See of Rome is vacant.
Pope St. Pius X condemned the modernist heresy as did other Popes before him.
It's not possible for a Pope to be a heretic because 1) heretics are not Catholic and being Pope requires you to be Catholic and 2) Christ promised the Gates of Hell would never prevail against Peter (the Pope). Which logically concludes that the Vatican II so-called Popes from John XXIII to Francis are anti-Popes (papal usurpers) never mind the fact that they are modernist heretics.
(post is archived)