Dictionary.net

• a nuclear weapon in which enormous energy is released by nuclear fission (splitting the nuclei of a heavy element (uranium 235 or plutonium 239))

2011 - English Dictionary Database
By DataStellar Co., Ltd

• a nuclear weapon in which enormous energy is released by nuclear fission (splitting the nuclei of a heavy element like uranium 235 or plutonium 239)

2006 - WordNet 3.0
By Princeton University