Meaning of president pro tempore

pres'ident pro tem'pore

Pronunciation: [key]
— U.S. Govt. U.S. Govt.
  1. a senator, usually a senior member of the majority party, who is chosen to preside over the Senate in the absence of the vice president.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.