Tsunami is a Japanese word which means 'harbour wave'. A tsunami is a large sea wave caused by the displacement of a large volume of water. They can be caused by earthquakes triggered by moving sections of the Earth's crust under the ocean.
In the last decade there have been a number of devastating tsunamis. Two large ones caused particularly extensive devastation: the Indian Ocean tsunami (26 December 2004) and the Japanese tsunami (11 March 2011).
The Indian Ocean tsunami of 2004 was caused by plates moving and slipping under the ocean.