Air Force One is not the name of one particular plane. Instead, it is the name given to any plane that the President of the United States is traveling on at a given moment. This is essentially a radio call name used to avoid confusion with other planes in the same area. Similarly, if the president is traveling on an Army aircraft, it is referred to as Army One, and if they are on a helicopter, it is called Marine One.
In modern usage, however, the term 'Air Force One' often refers to the custom-made plane typically associated with the president.
