ex cathedra

from WordNet (r) 3.0 (2006)

ex cathedra
    adv 1: with the full authority of the office; "the pope must
           speak ex cathedra for an encyclical to be infallible"

from The Collaborative International Dictionary of English v.0.48

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]