WMD - Acronym (Weapon of Mass Destruction): An implement of war utilized to inflict mass casualties. These weapons typically fall into three categories: Chemical, Biological, and Nuclear.
Chemical Weapon - Using a caustic, non-biological agent to harm living entities.
Biological Weapon - Using a living organism (disease, virii, etc.) to harm living entities.
Nuclear Weapon - Utilizing an uncontrolled nuclear fission reaction (atomic bomb) and/or radioactive materials ("dirty bomb") to cause destruction to living entities and inanimate objects.
Needless to say, the production, possession, trade, and use of WMDs is and will remain a topic of contention and controversy.
A "Neutron Bomb" is a fictional WMD. In theory, it uses a powerful (so far fictional) agent "Red Mercury" to cause a massive flux of energy. It causes no concussive blast, but annihilates all living organisms within a large radius.