Tsunami or etymologically meaning "big waves in the harbor", are large waves of water caused by disturbances on the seabed, such as earthquakes. This disturbance forms waves that spread in all directions with wave speeds reaching 600–900 km/hour. Initially these waves have a small amplitude (usually 30–60 cm) so they are not felt on the high seas, but their amplitude increases as they approach the coast.

On reaching the coast, the tsunami sometimes hits land in the form of a giant wall of water (especially in large tsunamis), but the more common form is a sudden rise in water level. The rise in water levels can reach 15–30 meters, causing flooding with currents of up to 90 km/h, extending several kilometers from the coast, and causing great damage and loss of life.

The most common cause of tsunamis are underwater earthquakes, especially those that occur in subduction zones with a magnitude of 7.0 or more on the moment magnitude scale. Other causes are landslides, volcanic eruptions, and the fall of large objects such as meteors into the water.

Geographically, almost all tsunamis occurred in the Pacific Ring of Fire and the Sumatra Trench in the Indian Ocean. Tsunami risk can be detected with a tsunami early warning system that observes large earthquakes and analyzes data on changes in sea level that occur afterward. If a tsunami risk is deemed to exist, the authorities can issue a warning or take action such as evacuation. The risk of damage can also be reduced by tsunami-resistant designs, such as building buildings with large spaces, and the use of reinforced concrete materials, as well as by educating the public on how to save themselves from a tsunami, such as the importance of evacuating and preparing emergency plans ahead of time.