The Collaborative International Dictionary of English v.0.48:

Nuncio \Nun"ci*o\, n.; pl. Nuncios. [It. nunzio, nuncio, fr.
   L. nuncius, nuntius, messenger; perh. akin to novus new, E.
   new, and thus, one who brings news. Cf. Announce.]
   [1913 Webster]
   1. A messenger. [Obs.] --Shak.
      [1913 Webster]

   2. The permanent official representative of the pope at a
      foreign court or seat of government. Distinguished from a
      legate a latere, whose mission is temporary in its nature,
      or for some special purpose. Nuncios are of higher rank
      than internuncios.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nuncio
    n 1: (Roman Catholic Church) a diplomatic representative of the
         Pope having ambassadorial status [syn: nuncio, papal
         nuncio]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "nuncio":
   Hermes, Iris, Mercury, Paul Revere, Pheidippides, ambassador,
   ambassadress, apostolic delegate, attache, career diplomat,
   carrier, chancellor, charge, commercial attache, commissionaire,
   consul, consul general, consular agent, courier, diplomat,
   diplomatic, diplomatic agent, diplomatic courier, diplomatist,
   emissary, envoy, envoy extraordinary, estafette, express,
   foreign service officer, go-between, internuncio, legate,
   message-bearer, messenger, military attache, minister,
   minister plenipotentiary, minister resident, plenipotentiary, post,
   postboy, postrider, resident, runner, secretary of legation,
   vice-consul, vice-legate



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

NUNCIO. The name given to the Pope's ambassador. Nuncios are ordinary or
extraordinary; the former are sent upon usual missions, the latter upon
special occasions.