A major league is a league that has be part of Major League Baseball throughout its history. Currently, it consists of the American League and National Leagues. In the past its has included the American Association, Union Association, and Players League in the 19th century, and the Federal League in the 20th century. It may also include the National Association of Professional Base Ball Players, a precursor to National League; however Major League Baseball does not recognize the league.
In a major move meant to counter decades of discrimination and oppression, Major League Baseball decided on December 16, 2020 to also recognize the Negro Leagues, in their heyday from 1920 to 1948, as another major league. This had been a de facto policy for a while - for example, the Hall of Fame had inducted a number of players based strictly on their performance in the Negro Leagues - but it was also long overdue. As Commissioner Rob Manfred put it: "All of us who love baseball have long known that the Negro Leagues produced many of our game's best players, innovations and triumphs against a backdrop of injustice. We are now grateful to count the players of the Negro Leagues where they belong: as Major Leaguers within the official historical record."
This decision affects seven different leagues: the first Negro National League (1920-1931), the Eastern Colored League (1923-1928), the American Negro League (1929), the East-West League (1932), the Negro Southern League (1932), the second Negro National League (1933-1948) and the Negro American League (1937-1948). The decision specifically pointed to the Seamheads Negro League Database as the foremost source of statistical information about these leagues. MLB mandated the Elias Sports Bureau, its official statistician, to work with leading historians and other experts to determine the effect of the decision on the official statistical record. An estimated 3,400 players were affected by the decision and were now considered big leaguers.
- Anthony Castrovince: "MLB adds Negro Leagues to official records", mlb.com, December 16, 2020. 
- Sean Forman and Cecilia M. Tan, eds.: The Negro Leagues Are Major Leagues: Essays and Research for Overdue Recognition, Baseball-Reference and SABR, Phoenix, AZ, 2021. ISBN ISBN 978-1-970159-63-9
- George Gmelch: Inside Pitch: Life in Professional Baseball, University of Nebraska Press, Lincoln, NE, 2006. (Originally published in 2001). ISBN 978-0-8032-7128-9
- Philip Lee: Black Stats Matter: Integrating Negro League Numbers into Major League Records, McFarland, Jefferson, NC, 2022. ISBN 978-1-4766-8834-3