WMD is a standard acronym for "Weapon of Mass Destruction". The term is often used synonymously with NBC or NBCR (Nuclear, Biological, Chemical, or Radiological) weapon. These are a class of weapons sharing certain important characteristics. The most important is that they are capable of inflicting extremely large numbers of casualties when used against civilian targets. Secondarily, they are usually of limited military use. This can be because casualties are long delayed from time of exposure or because counter-measures for protecting military targets are either cheap, making them useless against enemy military forces, or essentially impossible, leading to a deadlock in which opposing military forces benefit from bilaterally relinquishing their use.
Alternatively, the scope of the damage associated with using a weapon of mass destruction may be so great as to threaten the user. Chemical and radiological weapons are examples of the first class, as are those biological weapons which are either non-contagious or controllable through inoculation. Thermonuclear weapons and uncontainable plagues are examples of the second class. Another common feature of WMDs is that they typically destroy land or render it worthless for long periods, making them useless for purposes of conquest. These characteristics combine to make WMDs primarily useful as weapons of terror, either between nations who or between civilian terrorists and perceived oppressors who cannot plausibly be defeated by conventional means.
There has been a great deal of concern regarding the possibility of WMD terrorism in recent years, but so far all such terrorist attacks have been ineffective compared to expectations. The most lethal such attack was the use of nerve gas in the Tokyo subway by the Japanese doomsday cult Aum Shinri Kyo (Supreme Truth). This attack only killed 12 people, but 5500 were injured. As a general rule, the preparation of dispersion mechanisms for airborn toxins and microbes seems to be beyond the practical scientific capabilities of most of the engineers and scientists who choose to join cults and terrorist groups.
Unfortunately, there is some concern that rogue states may internally fund the development of WMDs for sale to terrorists. Even more unfortunately, the fall of the Soviet Union resulted in a large number of unemployed scientists with highly professional experience building such weapons. More optimistically, this has been the situation for almost a generation at this point and has not yet lead to any effective acts of WMD terrorism. By now, without adequate maintenance, most WMDs, and for that matter most scientists produced by the USSR have probably become less effective than they would have initially been, so this threat may be passing. In addition, since 9/11 there has been a great deal of effort to reduce vulnerability to such attacks through the development of sensors, protocols for harm containment and intelligence data integration. For unrelated reasons, disease containment technology has also improved. The generally successful containment of SARS in recent years and the development of antivirals such as tamiflu give us some basis for expecting that even the most terrifying of all possible WMDs, the genetically modified pandemic disease, may be becoming more manageable through countermeasures more rapidly than it becomes more threatening through the diffusion of the technologies required to create it.