The Collaborative International Dictionary of English v.0.48:

Catholicos \Ca*thol"i*cos\, n. [NL. See Catholic.] (Eccl.)
   The spiritual head of the Armenian church, who resides at
   Etchmiadzin, Russia, and has ecclesiastical jurisdiction
   over, and consecrates the holy oil for, the Armenians of
   Russia, Turkey, and Persia, including the Patriarchs of
   Constantinople, Jerusalem, and Sis.
   [1913 Webster]

   Note: The Patriarch of Constantinople is the civil head of
         the Armenians in Turkey.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Catholicos
    n 1: the ecclesiastical title of the leaders of the Nestorian
         and Armenian churches