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

Ambassador

An ambassador is the highest ranking diplomat who represents a nation and is usually accredited to a foreign sovereign or government, or to an international organization....

-in-residence who is usually limited to a country and/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.

Honorific Title

According to International Protocol Ambassadors and/or Ambassadors at Large are officially styled and addressed as His/Her Excellency, or Mr. Ambassador. The title may be abbreviated in official correspondence as H.E. ( Source The Guide of the Modern Diplomatic Protocol)

United States Ambassadors-at-Large

The US President

President of the United States

The President of the United States of America is the head of state and head of government of the United States. The president leads the executive branch of the federal government and is the commander-in-chief of the United States Armed Forces....

 appointed the first official specifically to bear the title of ambassador-at-large on March 12, 1949, under the authority vested in Article II, Section 2 of the U.S Constitution to deal with specific foreign policy issues which have been frequently, but not always, spelled out in their commissions.

Notable Ambassadors-at-Large

• H.E. Dame Nuala O'Loan
  Nuala O'Loan
  Nuala Patricia O'Loan, Baroness O'Loan, DBE is a noted public figure in Northern Ireland. She was the first Police Ombudsman in between 1999 and 2007. In July 2009, it was announced that she was to be appointed to the House of Lords. Consequently, she was raised to the peerage as Baroness O'Loan,...
  
   http://www.irishembassy.nl/home/index.aspx?id=43535, of Ireland. Roving Ambassador for Conflict Resolution and Special Envoy to Timor-Leste
• H.E. Suzan Johnson Cook
  Suzan Johnson Cook
  Dr. Suzan Denise Johnson Cook is the United States Ambassador-at-Large for International Religious Freedom, pastor, author and presidential advisor.-Biography:...
  
  , US Ambassador-at-Large for International Religious Freedom
• H.E. David Scheffer http://www.encyclopedia.com/doc/1P1-28878675.html, US Ambassador-at-Large for War Crime Issues
• H.E. Strobe Talbott
  Strobe Talbott
  Nelson Strobridge "Strobe" Talbott III is an American foreign policy analyst associated with Yale University and the Brookings Institution, a former journalist associated with Time magazine and diplomat who served as the Deputy Secretary of State from 1994 to 2001.-Early life:Born in Dayton, Ohio...
  
   http://findarticles.com/p/articles/mi_m1584/is_n27_v4/ai_13238133, US Ambassador-at-Large and Special Adviser to the Secretary on the New Independent States
• H.E. Dr Dumitru Ciausu http://romania-on-line.net/whoswho/CiausuDumitru.htm, Romanian Ambassador-at-Large
• H.E. Dr Bishhma K. Agnihotri http://www.bkagnihotri.org/dynamic/, India's Ambassador-at-Large for Non-Resident Indians and Persons of Indian Origin
• H.E. Javed Malik
  Javed Malik
  Javed Malik is Pakistan's Ambassador at Large. He is also a noted television journalist & Corporate Lawyer in addition to having business interests in London, Dubai and Pakistan. He is also the Editor in Chief of The Diplomat Magazine published from the UAE *...
  
  , Pakistani Ambassador-at-Large
• H.E. Juris Gulbis, Ambassador-at-Large of the Republic of Abkhazia to the Asia Pacific Region
• H.E. Kristiaan De Ley http://encyclo.wiki-site.com/index.php/Kristiaan_De_Ley, Liberian Ambassador-at-Large to UNESCO
• H.E. Hassan Abouyoub, Moroccan Ambassador-at-Large
• H.E. Mr. Petri Salo http://formin.finland.fi/Public/default.aspx?nodeid=16661&contentlan=2&culture=en-US, Finland's Roving Ambassador for South Caucasus (Armenia, Azerbaijan and Georgia).
• H.E. Dr. Eric Goosby
  Eric Goosby
  Ambassador Eric Goosby currently serves as the United States Global AIDS Coordinator. In this position Goosby directs the U.S. strategy for addressing HIV around the world and leads President Obama's implementation of the President's Emergency Plan for AIDS Relief . Goosby was sworn in on June 23,...
  
   http://www.state.gov/r/pa/ei/biog/125483.htm, United States Global AIDS Coordinator (US Ambassador-at-Large on AIDS)
• Georg von Habsburg http://www.economist.com/node/15551306?story_id=15551306&fsrc=rss, Ambassador-at-Large of Hungary.

Examples in the diplomatic corps of the US State Department include(d) :

• Ambassador-at-Large for War Crimes Issues
• Ambassador-at-Large for Counterterrorism
• Ambassador-at-Large for International Religious Freedom
  Ambassador-at-Large for International Religious Freedom
  The Ambassador-at-Large for International Religious Freedom is the ambassador-at-large who heads the Office of International Religious Freedom in the United States Department of State....
  
  
• Ambassador-at-Large for Global Women's Issues
• Ambassador-at-Large for HIV and AIDS