Wikipedia - Minor League Baseball

Minor League Baseball (MiLB) is a hierarchy of professional baseball minor leagues in the Americas that compete at levels below Major League Baseball (MLB) and help prepare players to join major league teams. Those teams which are affiliated with MLB teams operate under the Commissioner of Baseball within the scope of organized baseball. Several leagues, known as independent baseball leagues, consist of teams with no affiliation to MLB teams.

Starting with the 2021 season, the number of MLB-affiliated minor leagues with teams in the United States and Canada has been reduced to 11, with a total of 120 teams (four per each of the 30 MLB franchises, excluding rookie league teams). There is also an MLB-affiliated rookie league in the Dominican Republic.