Waivers (MLB)

Major League Baseball transactions are changes made to the roster of a major league team during or after the season. They may include waiving, releasing, and trading players, as well as assigning players to minor league teams.

Each Major League Baseball team maintains both a 25-man roster and a 40-man roster of players. Players on the 25-man roster are eligible to play in official major league games throughout the season. The 40-man roster includes the players on the 25-man roster plus as many as 15 players who are either on the team's seven- or 15-day disabled list, who are on paternity leave for up to three days, or who are in the franchise's farm teams in Minor League Baseball. From September 1 through the end of the regular season, any player on the 40-man roster (also referred to as the "expanded roster") is eligible to play in an official regular season game. Many young players make their Major League debuts in this way, as "September call-ups."

To be eligible for a team's postseason roster, a player must be on any of the following: (a) the 40-man roster, (b) the 60-day disabled list, or (c) the bereavement list as of August 31 at midnight ET. The only exception is that a player who is on the disabled list at the end of the regular season may be replaced on the postseason roster by any other player who was in the organization on August 31, with the approval of the Commissioner. All players on the team's roster during the season will be eligible to receive a World Series ring after the team wins the World Series.

Teams may trade only players currently under contract, except those players who have been drafted in the last year. From the end of the previous World Series through July, trades between two or more major league teams may freely occur at any time. In August, trades may only be made after all players in the trade clear waivers or are not on 40-man rosters. Players acquired after August 31 are ineligible for the postseason roster unless they replace an injured player. Unlike in the NFL, NHL, MLS, and the NBA, teams may not trade draft choices (with the exception of competitive balance draft selections, which can be traded), but may purchase the rights to Rule 5 Draft Picks.

...
Wikipedia