Baseball is a bat-and-ball sport played between two teams of nine players each. The goal is to score runs (run (baseball)) by hitting a thrown ball (Baseball (ball)) with a bat (baseball bat) and touching a series of four bases (base (baseball)) arranged at the corners of a ninety-foot square, or diamond. Players on one team (the batting team (Baseball rules#Batting team)) take turns hitting against the pitcher of the other team (the fielding team (Baseball rules#Fielding team)), which tries to stop them from scoring runs by getting hitters out (out (baseball)) in any of several ways. A player on the batting team can stop at any of the bases and later advance via a teammates hit (Hit (baseball)) 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 (inning (baseball)); 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 and the related rounders were brought by British and Irish immigrants to North America, where the modern version of baseball developed (History of baseball in the United States). By the late nineteenth century, baseball was widely recognized 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 sometimes referred to as hardball, in contrast to the derivative game of softball.

In North America, professional Major League Baseball (MLB) teams are divided into the National League (NL) and American League (AL). Each league has three divisions: East, West, and Central. Every year, the major league champion is determined by playoffs (Major League Baseball postseason) that culminate in the World Series. Four teams make the playoffs from each league: the three regular season division winners, plus one wild card (Wild card (sports)) team. Baseball is the leading team sport in both Japan and Cuba, and the top level of play is similarly split between two leagues: Japans Central League and Pacific League; Cubas West League and East League (Cuban National Series). In the National and Central leagues, the pitcher is required to bat, per the traditional rules. In the American, Pacific, and both Cuban leagues, there is a tenth player, a designated hitter, who bats for the pitcher. Each top-level team has a farm system (Farm team#baseball) of one or more minor league teams (Minor league baseball). These teams allow younger players to develop as they gain on-field experience against opponents with similar levels of skill.