The FBI defines WMD : http://www.fbi.gov/about-us/investigate/terrorism/wmd/wmd_faqs (the detailed version)

(the brief version)
Quote:
WMD is often referred to by the collection of modalities that make up the set of weapons: chemical, biological, radiological, nuclear, and explosive (CBRNE). These are weapons that have a relatively large-scale impact on people, property, and/or infrastructure.
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