A tsunami (Japanese: 津波 [tsɯnami], lit. ‘harbor wave’; English pronunciation: /suːˈnɑːmi/ or /tsuːˈnɑːmi/) or tidal wave is a series of water waves (called a tsunami wave train) caused by the displacement of a large volume of a body of water, such as an ocean or a large lake. Tsunamis are a frequent occurrence in Japan; approximately 195 events have been recorded. Due to the immense volumes of water and energy involved, tsunamis can devastate coastal regions. Casualties can be high because the waves move faster than humans can run.
Earthquakes, volcanic eruptions and other underwater explosions (detonations of nuclear devices at sea), landslides and other mass movements, bolide impacts, and other disturbances above or below water all have the potential to generate a tsunami.
(From Wikipedia, May 10th, 2010)