The Collaborative International Dictionary of English v.0.48:

Internuncio \In`ter*nun"ci*o\, n.; pl. Internuncios. [L.
   internuntius; inter between + nuntius, nuncius, messenger:
   cf. It. internunzio. See Nuncio.]
   [1913 Webster]
   1. A messenger between two parties. --Johnson.
      [1913 Webster]

   2. A representative, or charg['e] d'affaires, of the pope at
      a foreign court or seat of government, ranking next below
      a nuncio.
      [1913 Webster]

   Note: This title was formerly given also to the Austrian
         envoy at Constantinople.
         [1913 Webster]

WordNet (r) 3.0 (2006):

internuncio
    n 1: (Roman Catholic Church) a diplomatic representative of the
         Pope ranking below a nuncio

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

INTERNUNCIO. A minister of a second order, charged with the affairs of the
court of Rome, where that court has no nuncio under that title.