The Collaborative International Dictionary of English v.0.48:

bullpen \bull"pen`\ (b[.u]l"p[e^]n`), n. (Baseball)
   1. an open area in a baseball stadium, off the playing field,
      where pitchers may warm up by throwing a number of trial
      pitches before playing.
      [PJC]

   2. [collective] the relief pitchers on a baseball team.
      Contrasted with starters.
      [PJC]

WordNet (r) 3.0 (2006):

bullpen
    n 1: a place on a baseball field where relief pitchers can warm
         up during a game
    2: a large cell where prisoners (people awaiting trial or
       sentence or refugees or illegal immigrants) are confined
       together temporarily [syn: bullpen, detention cell,
       detention centre]