Pro tem is short for the Latin pro tempore, meaning temporarily or for the time being. In law, judge pro tem normally refers to a judge who is sitting temporarily for another judge or to an attorney who has been appointed to serve as a judge as a substitute for a regular judge. When an appeals justice is not available or there is a vacancy, a lower court judge is appointed Justice Pro Tem until a new Justice is appointed. Attorneys often serve as Judge Pro Tem in small claims cases. Pro tem judges are often used because there are too many cases for the regular judges to handle.
The United States Senate, according to the United States Constitution, (Article I), is required to choose a President Pro Tempore (or, "president for a time," often shortened to President Pro Tem), who presides over the Senate in the absence of the Vice President.