The Collaborative International Dictionary of English v.0.48:

Triumvir \Tri*um"vir\, n.; pl. L. Triumviri, E. Triumvirs.
   [L., fr. res, gen. trium, three + vir a man. See Three, and
   Virile.] (Rom. Antiq.)
   One of tree men united in public office or authority.
   [1913 Webster]

   Note: In later times the triumvirs of Rome were three men who
         jointly exercised sovereign power. Julius Caesar,
         Crassus, and Pompey were the first triumvirs;
         Octavianus (Augustus), Antony, and Lepidus were the
         second and last.
         [1913 Webster]

WordNet (r) 3.0 (2006):

triumvir
    n 1: one of a group of three sharing public administration or
         civil authority especially in ancient Rome