'Ex cathedra' definitions:

Definition of 'ex cathedra'

(from WordNet)
adverb
With the full authority of the office; "the pope must speak ex cathedra for an encyclical to be infallible"

Definition of 'Ex cathedra'

From: GCIDE
  • Cathedra \Cath"e*dra\, n. [L., fr. Gr. ? seat. See Chair.] The official chair or throne of a bishop, or of any person in high authority. [1913 Webster]
  • Ex cathedra [L., from the chair], in the exercise of one's office; with authority. [1913 Webster]
  • The Vatican Council declares that the Pope, is infallible "when he speaks ex cathedra." --Addis & Arnold's Cath. Dict. [1913 Webster]