Sedevacantism (derived from the Latin words sedes or "seat", and vacans or "vacant") is the position, held by a minority of Traditionalist Catholics, that the present occupant of the papal see is not truly Pope and that, for lack of a valid Pope, the see has been vacant since the death of Pope Pius XII in 1958. A tiny number of these claim the vacancy actually goes back to the death of Pope Pius X in 1914.
Sedevacantists believe that Paul VI (1963–1978), John Paul I (1978), John Paul II (1978–2005), Benedict XVI (2005–2013), and Francis (2013-) have been neither true Catholics nor true Popes, by virtue of allegedly having espoused the heresy of Modernism, or of having otherwise denied or contradicted solemnly defined Catholic dogmas. Some of them classify John XXIII (1958–1963) also as a Modernist antipope.
The term "sedevacantism" is derived from the Latin phrase sede vacante, which literally means "the seat being vacant", specifically in the context of the vacancy of the Holy See. This phrase is normally used between the death or resignation of a pope and the election of his successor. "Sedevacantism" as a term in English appears to date from the 1980s, though the movement itself is older.
Among those who maintain that the see of Rome, occupied by what they declare to be an illegitimate pope, was really vacant, some have chosen an alternative pope of their own, and thus in their view ended the vacancy of the see, and are known sometimes as "conclavists".