Air Force One

Air Force One is the official air traffic control designated call sign for a United States Air Force aircraft carrying the president of the United States. In common parlance, the term is used to denote U.S. Air Force aircraft modified and used to transport the president and a metonym for the primary presidential aircraft, VC-25, although it can be used to refer to any Air Force aircraft the president travels on.

The idea of designating specific military aircraft to transport the president arose during World War II when military advisors in the War Department were concerned about the risk of using commercial airlines for presidential travel. A C-54 Skymaster was then converted for presidential use; dubbed the Sacred Cow, it carried President Franklin D. Roosevelt to the Yalta Conference in February 1945 and was used for another two years by President Harry S. Truman.

Source: Wikipedia