WMD stands for weapons of mass destruction — weapons built to kill very large numbers of people at once, rather than one soldier at a time. In practice it covers three or four kinds: nuclear (bombs like the ones used in 1945), chemical (poison gas), biological (diseases used as a weapon), and sometimes radiological (an ordinary bomb that spreads radioactive material). You say the three letters one by one: W-M-D. The plural is usually WMDs, though careful writers sometimes leave it as WMD because the D already stands for a plural idea. Here is the part your students should really understand: this is a news and politics word, not a science word. It became famous around 2003, when the American and British governments said Iraq had WMD and used that as the reason to go to war; no such weapons were found. Because of that history, the word now carries weight. When a politician says WMD, listeners hear both "terrible danger" and, often, "is that true?" It is a word people argue about, not a neutral label — and that is exactly why it appears so often in newspapers and in reading tests.