An Apostolic Nunciature is the embassy of the Holy See in a foreign country. The nunciature protects the affairs of the Roman Catholic Church, not just the Vatican City, and also liases between the Holy See and the Roman Catholic Church in that country.
Some countries make the Nuncio Dean of the Diplomatic Corps (head of the group of ambassadors to its country). This is allowed by the international agreement about diplomats. Usually the Dean of the Diplomatic Corps is the longest serving ambassador in a country.
A pro-Nuncio was the name used for a Nuncio in countries which did not make a Nuncio Dean of the Diplomatic Corps automatically. The Vatican stopped using the title in 1991.
An Apostolic Delegate represented the Holy See to the church, but not the government, in another country.
- Vienna Convention on Diplomatic Relations 1961, Article 16