An ambassador-at-large is a diplomat, a secretary, or a minister of the highest rank who is accredited to represent a country and its people internationally.
Unlike an ambassador-in-residence, who is usually limited to a country or embassy, the ambassador-at-large is entrusted to operate in several usually neighboring countries, a region or sometimes hold a seat in an international organization like the United Nations and the other international organizations.
In some cases, an ambassador-at-large may even be specifically assigned a role to advise and assist the state or a 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 an ambassador-at-large.
[1] The US president appointed the first official who specifically bore the title of ambassador-at-large on 12 March 1949 under the authority vested in Article II, Section 2 of the US Constitution to deal with specific foreign policy issues, which have been frequently spelled out in their commissions.