What is Tsunami?

Hint: The word "tsunami" is a Japanese word for a geographical phenomenon. Earthquakes, volcanic eruptions, and other underwater explosions (like landslides, glacier calving, meteorite impacts) above or below water have the ability to produce a tsunami.

Complete answer: In Japanese “tsunami” means "harbour wave." A tsunami is produced by the movement of water, unlike regular ocean waves that are generated by wind or tides that are generated by the gravitational pull of the Moon and the Sun. A tsunami is a sequence of waves in a body of water caused by a large amount of water being displaced, usually in an ocean or a large lake.
When the seafloor suddenly deforms and vertically displaces the overlying water, tsunamis can be produced. Tectonic earthquakes are earthquakes associated with the crustal deformation of the Earth; the water above the deformed region is moved from its equilibrium location as these earthquakes occur underneath the sea. More precisely, a tsunami can be produced when, due to the vertical component of movement involved, thrust faults associated with convergent or destructive plate boundaries shift suddenly, resulting in water displacement.
Movement on normal (extensive) faults can also cause seabed displacement, but only the largest such events (typically related to bending in the outer swell of the trench) cause sufficient displacement to give rise to a major tsunami, such as the 1977 Sumba and 1933 Sanriku Events. With at least 230,000 people killed or missing in 14 countries bordering the Indian Ocean, the 2004 Indian Ocean tsunami was another among the worst natural disasters in human history.

Note: Because their wavelength is much longer, tsunami waves do not mimic regular undersea currents or sea waves. Instead of appearing as a breaking wave, a tsunami can initially resemble a quickly rising tide instead.
In general, tsunamis consist of a series of waves arriving in a so-called "shock train" with times varying from minutes to hours. Large events can produce wave heights of tens of metres. Although the effect of tsunamis is confined to coastal areas, they can be immense in their destructive strength, and they can affect entire ocean basins.