An umpire is a kind of referee, an official whose job on the baseball or softball field is enforcing the rules of the game. You might hear an umpire yell, "Strike three! You're out!"
When a conflict comes up during play, it's the umpire who mediates and ultimately makes an official decision. Umpire is also a verb, meaning to act as an umpire. The word comes from the Old French nonper or "odd number," which refers to a third person helping two others settle their differences.