The Collaborative International Dictionary of English v.0.48:

Nuclear weapon \Nu"cle*ar wea"pon\, n.
   A weapon of great explosive power, such as an atomic bomb
   or a hydrogen bomb, which depends for most of its explosive
   power on the release of energy from within atomic nuclei by a
   nuclear reaction. A fission weapon or a fusion weapon.
   The term includes atomic shells for cannon.
   [PJC]

WordNet (r) 3.0 (2006):

nuclear weapon
    n 1: a weapon of mass destruction whose explosive power derives
         from a nuclear reaction [syn: nuclear weapon, atomic
         weapon]