In the study of earth science, particularly, natural disasters (hazards), tsunamis (proncounced soo-nami) or tidal waves (different to tides) are massive sudden, unexpected and very large ocean waves. They are not associated with wind or storm surges.
They are caused by a sudden displacement (movement of water). This could be due to an undersea earthquake, an undersea landslide or meteor/asteroid strike. They are sometimes caused by icesheet crashing into bays too.
Tidal wave or tsunami?
There is sometimes a bit of an argument about what is the correct term to use. Scientists don't like the expression tidal wave because tides are caused by the moon. Although it is understandable to use the term since historically it looks like the tide suddenly rushes away and crashes back down as a large wave.
Origin of the word
The word tsunami comes from the Japanese word meaning "harbour wave" but does not have much to do with harbours. Although again historically people in harbour areas were/are most at risk of these devasting waves.
The German word for tidal wave is Flutwelle that literally means flood wave which is perhaps a good description.
The French word for tsunami is raz de maree, which means the power, cutting of the sea.
Seismic sea waves is also another term to describe tidal waves.
Here is the wikipedia description for reference, with a few more details
A tsunami (plural: tsunamis or tsunami; from Japanese: 津波, lit. "harbour wave" is a series of water waves caused by the displacement of a large volume of a body of water, generally an ocean or a large lake.
Earthquakes, volcanic eruptions and other underwater explosions (including detonations of underwater nuclear devices), landslides, glacier calvings, meteorite impacts and other disturbances above or below water all have the potential to generate a tsunami.
Tsunami waves do not resemble normal sea waves, because their wavelength is far longer. Rather than appearing as a breaking wave, a tsunami may instead initially resemble a rapidly rising tide, and for this reason they are often referred to as tidal waves.
Tsunamis generally consist of a series of waves with periods ranging from minutes to hours, arriving in a so-called "wave train". Wave heights of tens of metres can be generated by large events. Although the impact of tsunamis is limited to coastal areas, their destructive power can be enormous and they can affect entire ocean basins; the 2004 Indian Ocean tsunami was among the deadliest natural disasters in human history with at least 290,000 people killed or missing in 14 countries bordering the Indian Ocean.
The Greek historian Thucydides suggested in his late 5th century BC, History of the Peloponnesian War, that tsunamis were related to submarine earthquakes, but the understanding of a tsunami's nature remained slim until the 20th century and much remains unknown. Major areas of current research include trying to determine why some large earthquakes do not generate tsunamis while other smaller ones do; trying to accurately forecast the passage of tsunamis across the oceans; and also to forecast how tsunami waves would interact with specific shorelines.