Tsunami is the Japanese word for a phenomenon often called a tidal wave. In fact the two are quite distinct. (Tsu means harbour and nami a wave.)

A tidal wave occurs when the rising tide forces a mass of water along a narrow estuary or strait, causing a rise in water level. The tsunami is much more destructive. The usual cause of a tsunami is an earthquake or similar event.

The largest tsunami yet measured was 85 m (270 ft) tall and appeared off Ishigaki Island, east of Taiwan in 1971. Far more destructive was the tsunami which swept through Indonesia on 27 August, 1883. When the island volcano of Krakatoa exploded that morning it not only blew itself to pieces but created a huge tsunami. As it swamped the surrounding islands, the wave destroyed 163 villages, killed 36,380 people and carried a warship miles inland. The height of this massive wave is unknown. Fortunately such tsunamis are rare.