Source : Webster's Revised Unabridged Dictionary (1913)

Plenipotentiary \Plen`i*po*ten"ti*a*ry\, n.; pl.
   {Plenipotentiaries}. [LL. plenipotentiarius: cf. F.
   pl['e]nipotentiaire.]
   A person invested with full power to transact any business;
   especially, an ambassador or envoy to a foreign court, with
   full power to negotiate a treaty, or to transact other
   business.

Plenipotentiary \Plen`i*po*ten"ti*a*ry\, a.
   Containing or conferring full power; invested with full
   power; as, plenipotentiary license; plenipotentiary
   ministers. --Howell.

Source : WordNet®

plenipotentiary
     n : a diplomat who is fully authorized to represent his or her
         government