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]

The Collaborative International Dictionary of English v.0.48:

Plenipotentiary \Plen`i*po*ten"ti*a*ry\, a.
   Containing or conferring full power; invested with full
   power; as, plenipotentiary license; plenipotentiary
   ministers. --Howell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plenipotentiary
    n 1: a diplomat who is fully authorized to represent his or her
         government

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "plenipotentiary":
   absolute, all-powerful, almighty, ambassador, ambassadorial,
   ambassadress, apostolic delegate, attache, career diplomat,
   chancellor, charge, commercial attache, consul, consul general,
   consular, consular agent, diplomat, diplomatic, diplomatic agent,
   diplomatist, emissary, envoy, envoy extraordinary,
   foreign service officer, internuncio, legate, military attache,
   minister, minister plenipotentiary, minister resident, ministerial,
   nuncio, omnipotent, resident, secretary of legation, sovereign,
   supreme, unlimited, vice-consul, vice-legate



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PLENIPOTENTIARY. Possessing full powers; as, a minister plenipotentiary, is
one authorized fully to settle the matters connected with his mission,
subject however to the ratification of the government by which he is
authorized. Vide Minister.



The Devil's Dictionary (1881-1906):

PLENIPOTENTIARY, adj.  Having full power.  A Minister Plenipotentiary
is a diplomatist possessing absolute authority on condition that he
never exert it.