What is Bullpen?

Legal Definition
In baseball, the bullpen (or simply the pen) is the area where relief pitchers warm-up before entering a game. A team's roster of relief pitchers is also metonymically referred to as "the bullpen". These pitchers usually wait in the bullpen if they have not yet played in a game, rather than in the dugout with the rest of the team. The starting pitcher also makes his final pregame warm-up throws in the bullpen. Managers can call coaches in the bullpen on an in-house telephone from the dugout to tell a certain pitcher to begin his warm-up tosses.

Each team generally has its own bullpen consisting of two pitching rubbers and plates at regulation distance from each other. In most Major League Baseball parks, the bullpens are situated out-of-play behind the outfield fence. There are currently four MLB parks with bullpens in playable foul territory: Wrigley Field, AT&T Park, Oakland Coliseum and Tropicana Field.
-- Wikipedia
Legal Definition
An enclosure for bulls, an exercise area for baseball pitchers, the relief pitchers of a baseball team, an open-plan office area or a large cell in which prisoners are held before a court hearing.