The Collaborative International Dictionary of English v.0.48:

hit man \hit man\ n.
   1. A professional murderer, esp. one working for a criminal
      organization; also called torpedo. [Colloq.]
      [PJC]

   2. A slanderer working for political purposes to damage the
      reputation of an opponent; a hatchet man.
      [PJC]

WordNet (r) 3.0 (2006):

hit man
    n 1: a professional killer who uses a gun [syn: gunman,
         gunslinger, hired gun, gun, gun for hire,
         triggerman, hit man, hitman, torpedo, shooter]