The Collaborative International Dictionary of English v.0.48:

boom box \boom box\ n.
   a large portable casette or compact disk player, usually
   having an integrated radio receiver. It typically has two
   (stereophonic) speakers, and can be adjusted to play at a
   high sound intensity, from which the name comes.

   Syn: ghetto blaster.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

boom box
    n 1: a portable stereo [syn: ghetto blaster, boom box]