"Tsunami" is a Japanese word. "Tsu" means harbour and "nami" means wave. A "tsunami" is therefore a wave propagating in the ocean and when encountering the coasts, it can cause severe damages (bays, harbours...).
A tsunami often results from the sudden change in the topography of the ocean e.g. a seism due to plate tectonics or a landslide. But it can also be created by a meteorite impact. Explanation