Pro tem is short for the Latin term pro tempore which means a stand in or substitute. It is normal for the legal process on both the representative solicitor level and the hearing or sitting judge to have a course of action in place to follow if the situation arises where one person appointed to a role within a case cannot continue their role continuously and needs a break. Since the court case would not be allowed to break up, a substitute official is chosen to continue in the place of the person who could not.
This term is used widely within employment law as well to signify a temporary worker that is hired to fill in for a permanent worker for a set period of time. All municipal and governmental agencies also have this system as it is a requirement for a person to fill each job duty that is deemed necessary. The terms pro-tem and pro-tempore are interchangeable.