What is a weapon of mass destruction (WMD)?

"Weapons that are capable of a high order of destruction and/or being used in such a manner as to kill large numbers of people. Can be nuclear, chemical, biological, or radiological weapons but excludes the means of transporting or propelling the weapons where such means are a separable and divisible part of the weapon. Chemical Weapons and Biological Weapons need to be of a certain size to count as WMD - single chemical or biological artillery rounds would not be considered to be WMD, due to the limited damage they could produce."

"The term 'weapon of mass destruction' means any weapon or device that is intended, or has the capability, to cause death or serious bodily injury to a significant number of people through the release, dissemination, or impact of - 
   (A) toxic or poisonous chemicals or their precursors;
   (B) a disease organism; or
   (C) radiation or radioactivity."

"The term WMD generally applies to nuclear, biological, and chemical weapons; ballistic missiles; and, more recently, 'dirty bombs,' ordinary explosives containing some radioactive material. But this definition is too broad. Chemical weapons are not much more lethal than conventional explosives and hardly deserve the WMD label. Similarly, long-range ballistic missiles are especially destructive only if they have a nuclear or biological warhead, and so should not be considered a separate category. Dirty bombs cause local contamination and costly cleanup but not true mass destruction; they too should be given lower priority."

"Experts who have studied this subject during and after the Cold War accept that the definition has become more flexible since the days when weapons of mass destruction meant an attack by dozens of Soviet intercontinental ballistic missiles with nuclear warheads, capable of causing millions of deaths. Today the acronym WMD appears to mean different things to different people.

Experts say that battlefield artillery shells, mortars, short-range rockets or other systems fitted with nuclear, chemical, biological or radiological warheads can all be classed as weapons of mass destruction under the more liberal definition accepted today.

There are two common denominators that have survived the Cold War: by definition, today’s WMD systems in the hands of rogue or potentially hostile states have to be weapons fitted with nuclear, chemical or biological warheads, and the intention of the attacker, whether using short-range or long-range systems, would be to have a strategic impact."

"...[The term weapons of mass destruction] has a history in international diplomacy that extends back nearly 60 years. As one would expect of a term used in international agreements, it has an accepted meaning for use in disarmament negotiations and in defining treaty obligations accepted by the United States. That specific definition is clearly the one proposed to the United Nations in 1948 and used in subsequent disarmament negotiations. Hence, any definition of the term  weapons of mass destruction used as a matter of policy by the U.S. Government should be consistent with that one, effectively meaning either nuclear, biological, and chemical, or chemical, biological, radiological, or nuclear."