A trade is a type of transaction which occurs when two teams exchange players, future considerations or a player to be named later, money, draft picks or some combination thereof. In the major leagues, trades cannot be made after the trading deadline (normally on July 31st, although it has been set at other dates certain year), unless the players involved have cleared waivers. Players traded after August 31st are not eligible for the team's postseason roster. During the period of July 29-31, some teams out of contention trade their marquee players or players who are signed to large contracts for prospects or less expensive players. Another period of furious trading occurs around the annual winter meetings.
The first major league trade ever seems to have happened in 1886 in the American Association when Jack Boyle of the Cincinnati Red Stockings and $400 were exchanged for Hugh Nicol of the St. Louis Browns. There is, however, a minority view which says an earlier trade happened in 1885. Source: MLB article
A "blockbuster trade" is one in which particularly prominent players, or a large number of players are involved. Throughout baseball history, there have been famous, high-profile trades. Here are some (only key players may be mentioned):
- 1899: Honus Wagner, Fred Clarke, Rube Waddell, Deacon Phillippe, Tommy Leach and others for Jack Chesbro and others (part of the collapse of syndicate baseball and the contraction of the National League)
- 1900: An ageing Amos Rusie for a young Christy Mathewson.
- 1907: A trade of Ty Cobb for Elmer Flick does not go through.
- 1915: Joe Jackson comes to the Chicago White Sox for Braggo Roth and others.
- 1916: Tris Speaker for Sad Sam Jones.
- 1919: Babe Ruth is sold to the New York Yankees.
- 1926: Rogers Hornsby for Frankie Frisch.
- 1930: Goose Goslin for Heinie Manush.
- 1960: Rocky Colavito for Harvey Kuenn.
- 1964: Lou Brock for Ernie Broglio (famous mostly in retrospect).
- 1965: Frank Robinson for Milt Pappas and others.
- 1971: Joe Morgan and four others to the Cincinnati Reds for Lee May and others.
- 1990: Roberto Alomar and Joe Carter for Tony Fernandez and Fred McGriff
- 1992: Jeff Russell, Ruben Sierra and Bobby Witt for Jose Canseco
- 1998: Mike Piazza for Bobby Bonilla, Gary Sheffield and others. Piazza is then traded again eight days later.
- 2004: Alex Rodriguez ("A-Rod") and cash for Alfonso Soriano (largely a salary dump).
- 2012: Josh Beckett, Carl Crawford, Adrian Gonzalez and Nick Punto for five players
- 2020: Mookie Betts and David Price for Alex Verdugo (largely a salary dump, and other related trades fell through).
See also Largest Trade in Baseball History.
- Anthony Castrovince: "The 10 strangest trades in MLB history", mlb.com, May 2, 2020. 
- Manny Randhawa: "Most stunning offseason trades in MLB history", mlb.com, November 21, 2020.