A bullpen is an area of a baseball field that allows pitchers to warm up before entering a game. The bullpen is usually set to the side at one end of the team's dugout, but there is no rule for where it has to be. The bullpen often receives a lot of attention, especially toward the end of a game, as other pitchers may be called in to relieve the starter. Just like the regular field of play, the bullpen has a pitcher's mound and home plate.
There are a number of theories that suggest how the area got its name. Though it would be hard to verify, some seem to make quite a bit of sense. A bullpen is an area where bulls were kept before bullfights. In baseball, it may also get its name from Bull Durham tobacco, which may have often been advertised where the pitchers warm up. There are many other theories as well, some which seem far fetched and some that seem very plausible.
As there is no rule for where bullpens must be placed, other than they must be outside the field of play, they are located in many different areas. While they most commonly adjoin the dugout, bullpens at some stadiums are located behind the outfield fences. They may also be enclosed with a wall, or open to the existing field. When open to the existing field, there could be times when a live ball wanders into the bullpen, causing a greater chance for interference with pitchers and catchers who may be warming up. This adds yet another element to the game that many players may not be forced to deal with on a daily basis.
At some levels of baseball, communications between the team manager and bullpen happen through a closed-line telephone system. The manager in the dugout calls a pitching coach in the bullpen, informing that coach that he wants a certain pitcher, or perhaps more than one pitcher, to begin warming up. A warm-up catcher or two are also available to help get the pitcher ready to go into the game. At lower levels of the game, the manager may go directly to the bullpen to communicate with coaches and players.
Often, in addition to referring to the actual space, the bullpen may be a reference to all the pitchers who may be available to come into the game. For example, when a commentator says the team has an experienced bullpen, this is, obviously, not a reference to a warm-up area. Usually, the bullpen consists of six to eight pitchers, perhaps more. However, some of the pitchers sitting in the bullpen are starters on other days and may not be called upon to pitch except in extreme circumstances.