Vice President of the United States of America
The Vice President of the United States is the official who becomes President (or, in some cases, Acting President) in case of a vacancy in the presidency or the permanent or temporary disability of the President. The only official duty of the office is to preside over the U.S. Senate, with the authority to cast a tie-breaking vote. (In practice, though, the Vice President rarely attends Senate meetings, except on ceremonial occasions or when a tied vote is expected to occur. At most times, one or another member of the Senate actually wields the gavel.) The current Vice President is Mike Pence.
The procedures for presidential succession are spelled out in the 25th Amendment to the Constitution.