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]

WordNet (r) 3.0 (2006):

cathedra
    n 1: a throne that is the official chair of a bishop [syn:
         cathedra, bishop's throne]