A term in baseball that refers to the official complement of players on a baseball team who are available to participate in a game.
There are two types of rosters in Major League baseball: the 25-man roster, and the 40-man roster. Through the majority of the regular season, a baseball team is allowed to field a roster of 25 players. This includes the eight players in the starting lineup plus the starting pitcher, and any additional players are part of the team's reserve. Only the players on the official team roster are eligible to enter the field of play for any given game. Typically, the only manner in which a player who is not on the roster may play in a game is for one of the current members of the roster to be placed on the team's disabled list (DL).
After September 1, when the regular season is nearing its end, baseball rosters expand to a maximum of 40 players. At this point additional players, including those called up from the minor leagues, can be added to the roster, and are now able to participate in a game. The promotion of minor league players to the major leagues at this time is usually referred to as the "September Call-ups."