Major League Baseball (MLB) is a professional baseball organization, the oldest of the four major professional sports leagues in the United States and Canada. A total of 30 teams play in the National League (NL) and American League (AL), with 15 teams in each league. The NL and AL were formed as separate legal entities in 1876 and 1901 respectively. After cooperating but remaining legally separate entities beginning in 1903, the leagues merged into a single organization led by the Commissioner of Baseball in 2000.[7][8] The organization also oversees Minor League Baseball, which comprises 256 teams affiliated with the Major League clubs. With the World Baseball Softball Confederation, MLB manages the international World Baseball Classic tournament.

In 2000 the AL and NL were dissolved as legal entities, and MLB became a single, overall league de jure, similar to the National Football League (NFL), National Basketball Association (NBA) and National Hockey League (NHL)—albeit with two components called "leagues" instead of "conferences." The same rules and regulations are used in both leagues, with one exception: the AL operates under the designated hitter rule, while the NL does not.[22][23] This difference in rules between leagues is unique to MLB; the other sports leagues of the U.S. and Canada have one set of rules for all teams.