Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

triumvir
     n : one of a group of three sharing public administration or
         civil authority especially in ancient Rome
     [also: {triumviri} (pl)]