Tsunami is a Japanese term that describes a large seismically generated sea wave which is capable of considerable destruction in certain coastal areas, especially where underwater earthquakes occur.

In Japanese, "Tsunami" means "Harbor Wave" or "Wave In The Harbor" It is now internationally accepted as the term that defines a "Seismic Seawave."

In South America, the term "Maremoto" is frequently used to describe a Tsunami.
Tsunami is pronounced: (sue-NAM-ee)

Tsunami's have been incorrectly referred to as "tidal waves." A tidal wave is a non-technical term for a shallow water wave caused by the gravitational interactions between the Sun, Moon, and Earth (high water is the crest of a tidal wave and low water is the trough).

Tsunami's are formed by a displacement of water. This can come from the slippage of the boundaries between two tectonic plates, volcanic eruption, under-water earthquake, or even landslides.

Out in the open ocean, Tsunami's might only be 1 meter in height, but as it reaches the shore in shallow water, it can rise to heights of 15-30 meters or more. Think about how a normal wave comes into a shore: the water moves away from the shore and then comes crashing back. This movement "heightens" the destruction power of a Tsunami.

Tsunami's can also reach speeds ranging from 400 to 500+ miles per hour? about the same speed as a jet airliner.

The enormous energy that a Tsunami can possess allows it to travel across entire oceans. They often proceed as an ordinary gravity wave? having a 15 to 60 minute intervals.

From a destruction perspective, Tsunami's have cost not hundreds of thousands, but millions of human lives over the recorded history of Earth.
Sources: U.S. Geological Survey & Pacific Tsunami Warning Center