Governed internationally by the International Baseball Federation (IBAF), baseball is an offense/defense bat-and-ball team sport. Baseball, like softball, is unlike most other competitive sports in that the defense is given control of the ball. The number of players on the field at any given time is lopsided heavily in favor of the defense, which always has nine players on the field, while the offense has between one and four. Each team assigns nine defensive players [See position player] on the field of play to face the other team's (the "opponents") players who are engaged in batting and baserunning.

The action begins with a head-to-head battle between the pitcher and the batter, who is positioned in the batter's box. The batter's box is the place where the batter stands when ready to receive a pitch from the pitcher. The aim for the offense is to score more runs than the opponents runs by hitting a thrown ball from the pitcher with a bat and arriving safely to a counter-clockwise series of four bases: first, second, third and home plate. A run is scored when the runner advances past the three bases and returns "home", as home plate is set next to the batter's box.

Players on the batting team take turns hitting against the pitcher of the fielding team, which tries to prevent 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 both teams, beginning with the visiting team, constitutes an inning, and nine innings comprise a standard 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-18th century. This game was brought by immigrants to North America, where the modern version developed. By the late 19th 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, parts of East Asia, and has garnered interest in Europe.

In the United States and Canada, professional Major League Baseball (MLB) teams are divided into the National League (NL) and American League (AL), each with three divisions: East, West, and Central. The major league champion is determined by playoffs that culminate in the World Series. Five teams make the playoffs from each league: the three regular season division winners, plus two wild card teams. Baseball is the leading team sport in both Japan and Cuba, and the top level of play is similarly split between two leagues: Japan's Central League and Pacific League; Cuba's West League and East League. 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 of one or more minor league teams.