The Trade Deadline, which almost always* falls at 4 p.m. ET on July 31, is the last point during the regular season at which players can be traded from one club to another.
* Major League Baseball set the 2016 Trade Deadline for Monday, Aug. 1., in order to avoid having the deadline fall in the middle of the schedule of day games on Sunday, July 31. Pushing the deadline back one day to Aug. 1 -- when no games are scheduled to begin before 7 p.m. ET -- prevented players from being traded in the midst of active games.
2019 rule change
Prior to 2019, July 31 was referred to as the non-waiver Trade Deadline, and players could be traded after that date if they first cleared revocable trade waivers.
The player's original club had three options when a waiver claim was placed: It could either work out a standard trade with the claiming club (the two sides had 48 hours to agree to a deal), allow the player -- and all money remaining on his contract -- to go to the claiming club with no return or pull the player back off waivers. A player who was pulled back off waivers could be placed on trade waivers a second time, but at that point the waiver request became irrevocable. If a player passed through waivers unclaimed, he could then be traded to any club, free of restriction (though all 40-man-roster players in the trade had to clear waivers before being dealt).
Although trades could be completed after Aug. 31 under the old rules, the last day in August was sometimes colloquially referred to as the "waiver Trade Deadline," as players acquired after that date were ineligible to be added to the postseason roster by their new teams.
As of 2019, the July 31 Trade Deadline is the only trade deadline. Players may still be placed and claimed on outright waivers after July 31, but trades will no longer be permitted after that date. With regards to newly acquired players, the Aug. 31 postseason roster deadline remains in effect.
Trade Deadline example
Shortly before 4 p.m. ET on July 31, 2015, the Mets acquired outfielder Yoenis Cespedes from the Tigers in exchange for Minor League pitchers Michael Fulmer and Luis Cessa. Cespedes went on to bat .287/.337/.604 with 17 home runs in 57 games for the Mets, who ended up winning the National League East as well as the NL pennant.