The Collaborative International Dictionary of English v.0.48:

Plenipotentiary \Plen`i*po*ten"ti*a*ry\
   (pl[e^]n"[i^]*p[-o]*t[e^]n"sh[i^]*[asl]*r[y^]), 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.
   [1913 Webster]