A “tsunami” is a huge wave from the ocean caused by geological movements beneath the sea, mainly by earthquakes, but also by volcanic eruptions, the hit of a meteorite or a nuclear explosion. As it is rather a temporary rise of the sea level, it cannot be felt on the ocean, but only by the wave hitting the coast.
The most devastating tsunami ever hit South East Asia on 26/12/2004.
The word “tsunami” is of Japanese origin, meaning “harbor wave”. It derives from fishermen coming back from fishing on a quiet sea, and finding the harbor destroyed by the ocean.