William Joseph Klem, known as "the Old Arbitrator", was an American baseball umpire who worked in Major League Baseball from 1905 to 1941, spending his entire career in the National League (NL). He worked 18 World Series, which is a major league record. Klem was posthumously inducted into the Baseball Hall of Fame in 1953.

In baseball, the umpire is the person charged with officiating the game, including beginning and ending the game, enforcing the rules of the game and the grounds, making judgment calls on plays, and handling the disciplinary actions. The term is often shortened to the colloquial form ump. They are also sometimes addressed as blue at lower levels due to the common color of the uniform worn by umpires. In professional baseball, the term blue is seldom used by players or managers, who instead call the umpire by name. Although games were often officiated by a sole umpire in the formative years of the sport, since the turn of the 20th century, officiating has been commonly divided among several umpires, who form the umpiring crew. The position is analogous to that of a referee in many other sports.