A tsunami is a huge wave, or usually a series of waves, caused by something violent under the sea — most often an earthquake, sometimes a volcano or an undersea landslide. Out in deep water it is low and easy to miss; the danger comes when it reaches shallow water near land, where it piles up, floods far inland, and destroys everything in its path. Two things surprise learners. First, it is not one wave but several, and the first is often not the biggest — which is why people who go back to look after the first wave get killed. Second, English took this word straight from Japanese and kept the Japanese spelling, including that odd 'ts' at the start. Older English books called it a 'tidal wave', and you still hear that, but it is wrong: tides have nothing to do with it, and scientists and the news now say 'tsunami'. The word has a strong second life as a picture-word for any overwhelming rush of something: 'a tsunami of orders', 'a tsunami of criticism'. That use is fine in the news and in speech, but it is a strong image, so people avoid it in places where the real disaster is still raw. Since 2004 and 2011 the word carries real grief for many people, and a thoughtful speaker feels that.