Baseball is a bat-and-ball sport played between two teams of nine players each. The goal of baseball is to score runs by hitting a thrown ball with a bat and touching a series of four markers called bases arranged at the corners of a ninety-foot square, or diamond. Players on one team (the batting team) take turns hitting while the other team (the fielding team) 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 hope to score on a teammate's hit. The teams switch between batting and fielding whenever the fielding team gets three outs. One turn at bat for each team constitutes an inning; nine innings make up a professional game. The team with the most runs at the end of the game wins.

Baseball on the professional, amateur, and youth levels is popular in North America (particularly in the United States), Central America, parts of South America and the Caribbean, and parts of East Asia and Southeast Asia. The modern version of the game developed in North America, beginning in the eighteenth century. The consensus of historians is that it evolved from earlier bat-and-ball games, such as cricket and rounders, brought to the continent by British and Irish immigrants. By the late nineteenth century, baseball was widely recognized as the national sport of the United States. The game is sometimes referred to as hardball in contrast to the very similar game of softball.

In North America, professional Major League Baseball teams are divided into the National League (NL) and American League (AL). Each league has three divisions: East, West, and Central. Every year, the champion of Major League Baseball is determined by playoffs culminating in the World Series. Four teams make the playoffs from each league: the three regular season division winners, plus one wild card team. The wild card is the team with the best record among the non–division winners in the league. In the National League, the pitcher is required to bat, per the traditional rules. In the American League, there is a tenth player, a designated hitter, who bats for the pitcher. Each major league team has a "farm system" of minor league teams at various levels. These teams allow younger players to develop as they gain on-field experience against opponents with similar levels of skill.