Mistakenly called "tidal wave" the word tsunami is a Japanese word meaning, "harbor wave". A tsunami is a series of traveling ocean waves of extremely long length generated by disturbances associated primarily with earthquakes occurring below or near the ocean floor. Tsunamis are primarily the result of a vertical displacement of water and rank high on the scale of natural disasters.
Since 1850, tsunamis have been responsible for the loss of over 440,000 lives and billions of dollars of damage to coastal structures and habitats. One very large tsunami occurred in 2004.
At 7:59 am local time (0059Z; How to read Z-time), December 26, 2004, a extremely powerful underwater earthquake (magnitude 9.1 on the Richter scale) occurred just off the coast of the island of Sumatra in the Indian Ocean.
The click for movie showing the scope of the December 26, 2004 Sumatra tsunami as the waves moved around the world. More animations about this tsunami can be found at the NOAA Center for Tsunami Research.
Tied for fourth, a magnitude 9.0 (on the Richter scale) earthquake off the coast of Japan produced a tsunami that lead to an estimated 20,000 deaths on March 11, 2011.
Landslides (either into water or completely below the water surface) are also capable of generating tsunamis.
A tsunami, apparently triggered by a large underwater landslide from an earthquake, devastated the northwestern coast of Papua, New Guinea on July 17, 1998.
Three waves, the highest measuring more than 23 feet (7 meters) high, struck a six mile (10 km) stretch of coastline within ten minutes of the earthquake.
Portions of icebergs breaking or calving into the water can cause fjord tsunamis. (Fjords are narrow inlets of the sea with steep cliffs on either side.) But by far, earthquakes near "plate" boundaries produce the most tsunamis.