1897, from Japanese tsunami, from tsu "harbor" + nami "waves." tsunami in Science. tsunami. (ts -nä'mē) A very large ocean wave that is caused by an underwater earthquake or volcanic eruption and often causes extreme destruction when it strikes land.
Tsunami are actually waves caused by sudden movement of the ocean due to earthquakes, landslides on the sea floor, land slumping into the ocean, major volcanic eruptions or large meteorite impacts.