The Collaborative International Dictionary of English v.0.48:

magnetic monopole \magnetic monopole\ (Physics),
   a hypothetical subatomic particle having only one type of
   magnetic charge, the magnetic analogue of an electrically
   charged particle; it has only one magnetic pole instead of
   the two observed in all common magnetic object; -- it was
   still not observed as of 1998.
   [PJC]

WordNet (r) 3.0 (2006):

magnetic monopole
    n 1: a hypothetical particle with a single magnetic pole instead
         of the usual two