Twenty-fifth Amendment to the United States Constitution
The Twenty-fifth Amendment (Amendment XXV) to the United States Constitution says that if the President becomes unable to do his or her job, the Vice President becomes the President (Section 1) or Acting President (Sections 3 or 4).  This can happen for a short period of time, if the President is just sick or disabled for a short time.  It could also happen until the end of the President's term (the President's time in office), if the President died, resigned, or is "unable to discharge the powers and duties of his office".