Define "Apostolic Nuncio"?

In case you were wondering, the USCCB's papal blog on "What is an Apostolic Nuncio?":

The nuncio is the Vatican’s ambassador to the United States. (The United States and the Holy See have had full diplomatic relations since 1984.) Like other ambassadors, Archbishop Sambi represents the Holy See’s views on various issues to the U. S. government. In addition, he acts as a liaison between the Holy See and the Church in the United States. He announces the appointment of new bishops and often attends their ordination or installation as the Vatican’s representative. Since 2006, Archbishop Sambi has attended 35 such events!

The nuncio attends the semi-annual meetings of the United States Conference of Catholic Bishops and addresses the bishops at these meetings. In fact, Archbishop Sambi announced the Pope’s April 2008 visit at the November 2007 general meeting.

Archbishop Sambi is the second apostolic nuncio to the United States