Tsunami is a Japanese word which is combination tsu (meaning port) and nami (wave). But it is not a tidal wave. It is a powerful surge of water of the sea due to a subsea earthquake or eruption of a volcano. When water surges and heads for the coast, it can cause a massive flooding and can sweep away villages and towns on the coast.
Tsunami can be known in advance by volcanic roar, bubbling of water or sudden rise in temperature of the water. An advance warning is therefore possible.
The worst recorded tsunami happened on December 26, 2004 in South East Asia which killed thousands of people in the coastal countries like Thailand, India, Indonesia, Sri Lanka, Burma, Malaysia etc. Portugal’s capital Lisbon was totally destroyed in a tsunami in 1755.