A weapon of mass destruction (WMD) is a weapon which can kill large numbers of humans and/or cause great damage to man-made structures (e.g. buildings), natural structures (e.g. mountains) or the biosphere in general. The term covers several weapon types, including nuclear, biological, chemical (NBC) and, increasingly, radiological weapons. There is controversy over when the term was first used, either in 1937 (in reference to the aerial bombardment of Guernica, Spain) or in 1945 (with reference to nuclear weapons).

Following the atomic bombings of Hiroshima and Nagasaki, and progressing through the Cold War, the term came to refer more to non-conventional weapons. The phrase entered widespread usage in relation to the U.S.-led 2003 invasion of Iraq. Terms used in a military context include atomic, biological, and chemical warfare (ABC warfare), nuclear, biological, and chemical (NBC) after the invention of the hydrogen bomb, and chemical, biological, radiological, and nuclear (CBRN), recognizing the threat of subcritical radiological weapons.

Due to the indiscriminate impact of WMD, the fear of a WMD attack has shaped political policies and campaigns, fostered social movements, and has been the central theme of many films. Support for different levels of WMD development and control varies nationally and internationally. Yet understanding of the nature of the threats is not high, in part because of imprecise usage of the term by politicians and the media.

At that time, there were no nuclear weapons; biological weapons were already being researched by Japan,Biological Weapons Program - Japan (see Unit 731), and chemical weapons had seen wide use.

The application of the term to specifically nuclear and radiological weapons is traced by William Safire to the Russian phrase oruziye massovovo porazheniya. He credits James Goodby (of the Brookings Institution) with tracing what he considers the earliest known English-language use soon after the nuclear bombing of Hiroshima and Nagasaki (although it is not quite verbatim): a communique from a November 15, 1945 meeting of Harry Truman, Clement Attlee and Mackenzie King (probably drafted by Vannevar Bush ? or so Bush claimed in 1970) referred to "weapons adaptable to mass destruction". That exact phrase, says Safire, was also used by Bernard Baruch in 1946 (in a speech at the United Nations probably written by Herbert Bayard Swope)."Weapons of Mass Destruction", New York Times Magazine, April 19, 1998, p.22. Accessed online 24 February 2007. The same phrase found its way into the UN resolution to create the Atomic Energy Commission (predecessor of the International Atomic Energy Agency (IAEA)), which used the wording "?atomic weapons and of all other weapons adaptable to mass destruction".

An exact use of this term was given in a lecture "Atomic Energy as an Atomic Problem" by J. Robert Oppenheimer. The lecture was delivered to the Foreign Service and the State Department, on September 17th, 1947. The lecture is reprinted in The Open Mind (New York: Simon and Schuster, 1955). "It is a very far reaching control which would eliminate the rivalry between nations in this field, which would prevent the surreptitious arming of one nation against another, which would provide some cushion of time before atomic attack, and presumably therefore before any attack with weapons of mass destruction, and which would go a long way toward removing atomic energy at least as a source of conflict between the powers."

An early use of the exact phrase in an international treaty was in the Outer Space Treaty of 1967, however no definition was provided.