A very timely question. According to a pertinent article at Wikipedia, a weapon of mass destruction (WMD) is a weapon that can kill and bring significant harm to a large number of humans (and other life forms) and/or cause great damage to man-made structures (e.g. buildings), natural structures (e.g. mountains), or the biosphere in general.
The term "weapon of mass destruction" was first attributed to one Cosmo Gordon Lang, Archbishop of Canterbury, in 1937 in reference to the aerial bombardment of Guernica, Spain:
? Who can think at this present time without a sickening of the heart of the appalling slaughter, the suffering, the manifold misery brought by war to Spain and to China? Who can think without horror of what another widespread war would mean, waged as it would be with all the new weapons of mass destruction?
� 1998 -