In baseball, the bullpen is the area where pitchers warm-up before entering a game. Depending on the ballpark, it may be situated in foul territory down the baselines or just beyond the outfield fence. Also, the group of relief pitchers for a team is collectively referred to as the bullpen. These relievers usually wait in the bullpen when they have yet to play in a game, rather than in the dugout with the rest of the team.

The origin of the term bullpen is debated with no one theory gaining unanimous, or even substantial, prevalence over another. The term first appeared in wide use shortly after the turn of the 20th century [1] and has been used since in roughly its present meaning. Some of the most common theories are:

1. The bullpen symbolically represents the fenced in area of a bull's pen, where bulls wait before being sent off to the slaughter. The relief pitchers are the bulls and the bullpen represents their pen.
2. Late arrivers to ball games in the late 19th century were cordoned off into standing room only areas in foul territory. Because the fans were herded like cattle, this area became known as the bullpen, a designation which was later transferred over to the relief pitchers who warmed up there.
3. At the turn of the century, outfield fences were often adorned with advertisements for Bull Durham Tobacco . Since relievers warmed up in a nearby pen, the term bullpen was created.
4. In the 1800s, jails and holding cells were nicknamed bullpens, in respect of many police officers' bullish features -- strength and a short temper. The term was later applied to bullpens in baseball.
5. Casey Stengel suggested the term might have been derived from managers getting tired of their relief pitchers "shooting the bull " in the dugout and were therefore sent elsewhere, where they wouldn't be a bother to the rest of the team -- the bullpen. How serious he was when he made this claim is not clear.