cathedra
Cath"e*dra, n. Etym: [L., fr. Gr. Chair.] Defn: The official chair or throne of a bishop, or of any person in high authority. Ex cathedra Etym: [L., from the chair], in the exercise of one's office; with authority. The Vatican Council declares that the Pope, is infallible "when he speaks ex cathedra." Addis & Arnold's Cath. Dict.