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.