A retired number is a uniform number which has been taken out of circulation by a team and will no longer be issued to any other player. This is done to honor a player who was an icon for the team and wore that number, or more rarely one who died under tragic circumstances. Numbers can also be retired in honor of other uniformed personnel (a manager or a coach), or for other persons associated with the team, in which case a number is assigned even if that person never wore a uniform featuring the number (an example of this is number 83, retired by the Montreal Expos in honor of owner Charles Bronfman, or number 26 retired by the Los Angeles Angels in honor of Gene Autry).
A number can be retired immediately, either formally or informally, after its honoree's retirement, or it can be done many years later after it has been worn by others. The important thing is that once a number has been retired, it will no longer be issued. As a result, there are a few cases of numbers retired in honor of two different players with the same team.
Numbers are usually retired by a specific team, but there is one exception: number 42 is retired throughout Major League Baseball in honor of Jackie Robinson and his role in breaking baseball's color line. This was decided by Commissioner Bud Selig in 1997, on the 50th anniversary of Robinson's major league debut, although players who already wore the uniform - many of them in Robinson's honor - were allowed to keep wearing it. The most famous of these was Mariano Rivera who continued to wear the number until his retirement at the end of the 2013 season. The New York Yankees then retired the number in honor of both Robinson and Rivera. There is a movement to also retire number 21 throughout baseball, in honor of Roberto Clemente, although this has not been successful so far.
The first number to be retired was that of Lou Gehrig, number 4, following his forced retirement due to amyotrophic lateral sclerosis in 1939. This came only ten years after the Yankees had become the first team to issue numbers to their players, in 1929. The Yankees remain the team with the most retired numbers, with 21, honoring 22 different players (number 8 honors both Yogi Berra and Bill Dickey). In fact, every single-digit number (except for the non-standard 0) has been retired by the Yankees now that Derek Jeter's number 2 has been retired.
A number can be unretired. This has happened when a franchise has relocated: the numbers retired by the Expos were re-issued by the Washington Nationals after the team's move in 2005. This can also be the result of a decision by the team. The Miami Marlins unretired number 5, which had been retired in honor of original owner Carl Barger, who had died before the team played its first game. Presumably, a team could also at some point decide to unretire a number if terribly damaging information were to emerge about the person who had been originally honored.
For a list of retired numbers, see the individual articles about major league teams, and those about individual numbers.
- Will Leitch: "Each team's jersey number likely to be retired next", mlb.com, August 12, 2018.