An apostolic nuncio is an ecclesiastical diplomat, serving as an envoy or a permanent diplomatic representative of the Holy See to a state or to an international organization. A nuncio is appointed by and represents the Holy See, and is the head of the diplomatic mission, called an apostolic nunciature, which is the equivalent of an embassy. The Holy See is legally distinct from the Vatican City or the Catholic Church. In modern times, a nuncio is usually an archbishop.

The Holy See, also called the See of Rome, Petrine See or Apostolic See, is the jurisdiction of the pope in his role as the Bishop of Rome. It includes the apostolic episcopal see of the Diocese of Rome, which has ecclesiastical jurisdiction over the worldwide Catholic Church and sovereignty over the city-state known as the Vatican City. As the supreme body of government of the Catholic Church, the Holy See enjoys the status of a sovereign juridical entity under international law.