An Apostolic Nunciature is a top-level diplomatic mission of the Holy See, equivalent to an embassy.
The head of the Apostolic Nunciature is called nuncio. A nuncio is an ecclesiastical diplomatic title, derived from the ancient Latin nuntius, meaning messenger. A papal nuncio (officially known as an Apostolic nuncio) is a permanent diplomatic representative (head of diplomatic mission) of the Holy See to a state or international organization (e.g., the Arab League), having the rank of an ambassador extraordinary and plenipotentiary, usually with the ecclesiastical rank of titular archbishop.
In addition, the nuncio serves as the liaison between the Holy See and the Church in that particular nation, supervising the diocesan episcopate (usually a national conference of bishops which has its own chairman, usually the highest ranking (arch)bishop, especially if his seat carries the title of primate or he has individually been created a cardinal) and has an important role in the selection of bishops.