An ambassador is an official envoy, especially a high-ranking diplomat who represents a state and is usually accredited to another sovereign state or to an international organization as the resident representative of their own government or sovereign or appointed for a special and often temporary diplomatic assignment. The word is also often used more liberally for persons who are known, without national appointment, to represent certain professions, activities and fields of endeavor such as sales.
An ambassador is 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 vehicles are generally afforded diplomatic immunity in the host country. Under the Vienna Convention on Diplomatic Relations, an ambassador has the highest diplomatic rank. Countries may choose to maintain diplomatic relations at a lower level by appointing a charge d'affaires in place of an ambassador.
The foreign government to which an ambassador is assigned must first approve the person. In some cases, the foreign government might reverse its approval by declaring the diplomat a persona non grata, i.e. an unacceptable person. This kind of declaration usually results in recalling the ambassador to their home nation. In accordance with the Congress of Vienna of 1815 and the 1961 Vienna Convention on Diplomatic Relations, the ambassador and embassy staff are granted diplomatic immunity and personal safety while living abroad.
Due to the advent of modern technologies, today's world is a much smaller place in relative terms. With this in mind, it is considered important that the nations of the world have at least a small staff living in foreign capitals in order to aid travelers and visitors from their home nation. As an officer of the foreign service, an ambassador is expected to protect the citizens of their home country in the host country.
The rise of the modern diplomatic system was a product of the Italian Renaissance (from around AD 1300). The use of ambassadors became a political strategy in Italy during the 17th century. The political changes in Italy altered the role of ambassadors in diplomatic affairs. Because many of the states in Italy were small in size, they were particularly vulnerable to larger states. The ambassador system was used to disperse information and to protect the more vulnerable states.
The use of ambassadors today is widespread. States and non-state actors use diplomatic representatives to deal with any problems that occur within the international system. Ambassadors now normally live overseas or within the country in which it is assigned to for long periods of time so that they are acquainted with the culture and local people. This way they are more politically effective and trusted, enabling them to accomplish goals that their host country desires.
The Vienna Convention on Diplomatic Relations of 1961 formalized the system and is the set of international legislation in use nowadays. According to it, ambassadors are diplomats of the highest rank, formally representing their head of state, with plenipotentiary powers (i.e. full authority to represent the government). In modern usage, most ambassadors on foreign postings as head of mission carry the full title of Ambassador Extraordinary and Plenipotentiary. The distinction between extraordinary and ordinary ambassadors was common when not all ambassadors resided in the country to which they are assigned, often serving only for a specific purpose or mission.
Ambassadors carry formal letters of credence from their head of state, addressed to the host country's head of state. Because many Commonwealth countries have the same head of state, the accreditation of a High Commissioner is in the form of a simple and often informal letter of introduction from one head of government (Prime Minister) to that of another. The difference in accreditation is also reflected in the formal titles of envoys to foreign and Commonwealth states: e.g., British High Commissioners are formally titled "The High Commissioner for Her Majesty's Government in the United Kingdom", whereas British Ambassadors to foreign countries are known as "Her Britannic Majesty's Ambassador".
In some countries, a former ambassador may continue to be styled and addressed as ambassador throughout their life (in the United States, "Mr. Ambassador" or "Madam Ambassador" may be heard). In other countries, ambassador is a title that accrues to its holder only with respect to a specific position, and may not be used after leaving or beyond the position. Some countries do not use the term while an ambassador is in the home country, as the office holder is not an ambassador there; for example, a Canadian ambassador while in Canada is not generally addressed as ambassador, although they may be referred to as "Canadian ambassador to ..."; that is, with reference to a specific job function; the person is addressed or styled as ambassador only while holding such office.