A rookie is a baseball player playing his first season in the circuit. The term is most frequently used for a player in his first season in the major leagues, although minor league circuits also have their rookies.
The term is originally a military one, an abbreviation of the word "recruit". It gained popularity during the Civil War and then was applied to other spheres of life, including baseball. Baseball has long recognized that first-year players face a special challenge, as they must not only perform at the highest level of the game, but also learn about a whole set of rival players' tendencies and the quirks of various ballparks, while adapting to a different way of life. It has been traditional for veteran ballplayers to treat rookies with a measure of contempt, with hazing or initiation rituals being common.
Baseball instituted a Rookie of the Year Award in 1947. The definition of a rookie has been tweaked over the years, to reflect the fact that a large number of ballplayers get their first taste of the major leagues during September call-ups, when rosters are expanded from the normal 25 players, or during brief cups of coffee replacing an injured teammate or helping out a tired team. Thus, a player's "rookie season" is not necessarily the one in which he makes his major league debut, but the one when he first sees substantial playing time (defined as more than 40 games or 40 innings pitched) - provided his cumulative time from previous seasons has not brought him over these thresholds.
- Appalachian League (1963-present)
- Arizona League (1988-present)
- Dominican Summer League (1985-present)
- Gulf Coast League (1964-present); known as Sarasota Rookie League in 1964 and Florida Rookie League in 1965
- Pioneer League (1963-present)
Past rookie leagues have included:
- Cocoa Rookie League (1964)
- Florida East Coast League (1972)
- Mexican Rookie League (1968)
- Venezuelan Summer League (1997-2015)
- Michael Clair: "Rookie greats who never reached that level again", mlb.com, June 22, 2020. 
- Dave Zeman and David Nemec: The Baseball Rookies Encyclopedia, Potomac Books, Inc., Dulles, VA, 2004.