The Collaborative International Dictionary of English v.0.48:

Triumvirate \Tri*um"vi*rate\, n. [L. triumviratus: cf. F.
   triumvirat.]
   1. Government by three in coalition or association; the term
      of such a government.
      [1913 Webster]

   2. A coalition or association of three in office or
      authority; especially, the union of three men who obtained
      the government of the Roman empire.
      [1913 Webster]

WordNet (r) 3.0 (2006):

triumvirate
    n 1: a group of three men responsible for public administration
         or civil authority