Baseball is a bat-and-ball sport played flanked by two teams of nine players each. The goal is to score runs by hitting a thrown ball with a bat and touching a series of four bases agreed at the corners of a ninety-foot square, or diamond.

Players on one team (the batting team) take turns hitting against the pitcher of the additional team (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 extra means. The teams switch between batting and fielding whenever the fielding team records three outs. One turns 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 premature form of baseball was being played in England by the mid-eighteenth century.

This game and the related rounders were bring by British and Irish immigrants to North America, where the modern version of baseball developed. By the late nineteenth century, baseball was widely documented as the national sport of the United States. Baseball on the professional, amateur, and youth levels is now popular in North America, parts of Central and South America and the Caribbean, and parts of East Asia.

The game is from time to time referred to as hardball, in contrast to the derivative game of softball.