Wikipedia
An ambassador-at-large is a diplomat of the highest rank or a minister who is accredited to represent his country 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 European Union. 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.
Usage examples of "ambassador-at-large".
A television set was rolled in and turned on, the American ambassador-at-large and the Canadian Foreign Service Officer watching in fear and in silence.
Promoted six years ago to the post he currently holds: ambassador-at-large with special duties.
Within two hours, Hermione Chatham-Smythe, ambassador-at-large, and the premier of Ramor's planetary government had affixed their signatures to a treaty that once again allowed Man to deal commercially with the inhabitants of the Ramorian system.