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 his actual name in order to show respect (and to reflect the fact that MLB umpires do not always wear blue). 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.

In a game officiated by two or more umpires, the umpire in chief is the umpire who is in charge of the entire game. This umpire calls balls and strikes, calls fair balls and foul balls short of first/third base, and makes most calls concerning the batter or concerning baserunners near home plate. If another umpire leaves the infield to cover a potential play in foul ground or in the outfield, then the plate umpire may move to cover a potential play near second or third base. (The umpire-in-chief should not be confused with the crew chief, who is often a different umpire; see below.) In the event that an umpire is injured and only three remain, generally the second base position will be left vacant.

Baseball is a bat-and-ball sport played between two teams of nine players each. The aim is to score runs by hitting a thrown ball with a bat and touching a series of four bases arranged at the corners of a ninety-foot diamond. Players on the batting team take turns hitting against the pitcher of the fielding team, which tries to stop them from scoring runs by getting hitters out in any of several ways. A player on the batting team can stop at any of the bases and later advance via a teammate's hit or other means. The teams switch between batting and fielding whenever the fielding team records three outs. One turn at bat for each team constitutes an inning and nine innings make up a professional game. The team with the most runs at the end of the game wins.

Evolving from older bat-and-ball games, an early form of baseball was being played in England by the mid-eighteenth century. This game was brought by immigrants to North America, where the modern version developed. By the late nineteenth century, baseball was widely recognized as the national sport of the United States. Baseball is now popular in North America, parts of Central and South America and the Caribbean, and parts of East Asia.