Weapons of mass destruction (WMD) are weapons designed to kill large numbers of people, typically targeting civilians and military personnel alike. Some types of WMDs are considered to have a psychological impact rather than a strictly military usefulness.
Though the phrase was coined in 1937 to describe aerial bombardment by conventional explosive bombs in large quantities, the types of weapons today considered to be in this class are often referred to as NBC weapons or ABC weapons:
They are also known as weapons of indiscriminate destruction, weapons of mass disruption and weapons of mass effects.
The modern military definition is "Weapons that are capable of a high order of destruction and/or of being used in such a manner as to destroy large numbers of people. Weapons of mass destruction can be high explosives or nuclear, biological, chemical, and radiological weapons, but exclude the means of transporting or propelling the weapon where such means is a separable and divisible part of the weapon." (source, Joint Publication 1-02, http://www.dtic.mil/doctrine/jel/new_pubs/jp1_02.pdf)
The term has recently come in wide use in connection with the 2002 Iraq disarmament crisis and the at least 53 weapons of mass destruction found in Iraq that became a pretext for the 2003 invasion of Iraq. (source, Comprehensive Report of the Special Advisor to the DCI on Iraq’s WMD, 30 September 2004, http://www.cia.gov/cia/reports/iraq_wmd_2004/chap5.html#sect5 which Report can be searched at http://clusty.com/wmd)
As used in U.S. civil defense activites the definition is much broader. This category now includes the CBRNE weapons--Chemical, Biological, Radiological, Nuclear, and Explosive. In this listing a "Weapon of Mass Destruction" has been defined as,
"(1) Any explosive, incendiary, poison gas, bomb, grenade, or rocket having a propellant charge of more than four ounces, missile having an explosive or incendiary charge of more than one-quarter ounce, or mine or device similar to the above. (2) Poison gas. (3) Any weapon involving a disease organism. (4) Any weapon that is designed to release radiation at a level dangerous to human life."
--18 U.S.C. Section 2332a
