President pro tempore
A President pro tempore is an officer of a body who serves as President when the President is unavailable.
In the United States Senate, this is a Constitutionally recognized officer of the Senate who presides over the chamber in the absence of the Vice President. The President Pro Tempore (or, "president for a time") is elected by the Senate and is, by custom, the Senator of the majority party with the longest record of continuous service.
-  US Senate Reference