An ambassador is the highest ranking diplomat who represents their country.

Ambassadors are usually accredited to a foreign sovereign or government, or to an international organization, to serve as the official representative of their country.

Sometimes countries also appoint highly respected individuals as Ambassador at Large who are assigned specific responsibilities and they work to advise and assist their governments in a given area.

In everyday usage it applies to the ranking government representative stationed in a foreign capital. The host country typically allows the ambassador control of specific territory called an embassy, whose territory, staff, and even vehicles are generally afforded diplomatic immunity in the host country.

The senior diplomatic officers among members of the Commonwealth of Nations are known as High Commissioners, who are the heads of High Commissions. Representatives of the Holy See are known as Papal or Apostolic Nuncios.

Resident Coordinator of UN system are accredited to the Head of State and have the same rank.

Historically, officials representing their countries abroad were termed ministers, but this term was also applied to diplomats of the second rank. The Congress of Vienna of 1815 formalized the system of diplomatic rank under international law:

Ambassadors are ministers of the highest rank, with plenipotentiary authority to represent their head of state. In modern usage, most Ambassadors on foreign postings as head of mission carry the full title of Ambassador Extraordinary and Plenipotentiary. "Ordinary" Ambassadors and non-plenipotentiary status are rarely used, although they may be encountered in certain circumstances.

Resident Ambassador

Moreover, a Resident Ambassador is one who resides within the country to which (s)he is accredited. A Non-Resident Ambassador does not reside within the country to which (s)he is accredited but lives in a nearby country. Thus, a resident ambassador to a country might at the same time also be a non-resident ambassador to one or more other countries; this may be phrased as "Ambassador to" ... "with concurrent accreditation to" another country.

Ambassador at Large

Ambassador-at-Large is a Diplomat of the Highest rank and/ or a Minister who is accredited to represent his country. But unlike the Resident Ambassador (who is usually limited to a country)and/or embassy, the Ambassador-at-large is entrusted to operate in several usually neighbouring countries, a Region or sometimes a seat of international organizations like the United Nations/ European Union. In some cases an Ambassador at Large may even be specifically assigned a role to Advise and Assist the State or Government in particular issues. Historically, Presidents or Prime Ministers have designated special diplomatic envoys for specific assignments, primarily overseas but sometimes also within the country as Ambassadors-at-Large.