Catholic Church
The exercise of infallibility takes several forms (see Catechism of the Catholic Church, no. 890-891, citing Lumen Gentium, 25). One form is exercised by the Pope when speaking ex cathedra (literally, “from the chair” of St. Peter, in his authority as pope) on matters of faith or morals, even if he does so without the support of the bishops. This is an exercise of the Extraordinary Magisterium of the Church. While the pope has always held the power to exercise the Extraordinary Magisterium by speaking ex cathedra, the actual occurrence of an ex cathedra statement is quite rare. It is generally understood to have only occurred twice: Pope Pius IX’s definition of the dogma of Mary’s Immaculate Conception in 1854 and Pope Pius XII’s definition of the dogma of Mary’s Assumption in 1950. In both of these cases, the Pope was not teaching something new. Rather, he was confirming and clarifying something that the Church had already believed as part of God’s revelation.