Waterboarding is a method of torture. The victim is tied down with their head lower than their feet, a cloth is put over their face, and water is poured onto it. The water blocks the air, so the person's body believes it is drowning and panics, even though they are not actually under water. It causes extreme fear and suffering, and it can kill. It is used to force people to talk — usually prisoners, usually during questioning.

The word is almost always heard in one particular context: news and argument about how the American CIA treated prisoners after the September 11 attacks. Officials at the time called it an 'enhanced interrogation technique', which was a deliberately mild, bureaucratic phrase; human rights organisations, and later most governments and courts, called it torture. So when you meet this word in a newspaper, notice that the writer's choice of surrounding words is telling you their position. It is a heavy, serious word, not for casual use.

Grammatically it works two ways. As a noun it is the practice itself — 'waterboarding is illegal'. It is also the -ing form of the verb `waterboard` — 'he says he was waterboarded three times'. The passive form with `was waterboarded` is very common, because the sentence is nearly always about what was done to someone.