The word "tsunami" is actually a Japanese word. The first part, tsu, means harbor and nami means wave. So, in its original usage, tsunami meant a harbor wave. However, the term is now used to refer to waves caused by seismic (earthquake) or volcanic activity in or near the ocean floor. When there is an undersea disturbance, waves form and travel away from the center of the disturbance kind of like when you throw a rock in a pond. These waves can travel as fast as 450 miles per hour. Deep in the ocean, tsunami can pass undetected under ships. However, as they approach land, the water becomes more shallow and they rise up and crash on the shore. Tsunami can be very dangerous. They can damage or destroy coastal towns and villages. (Note: In the image above, notice the man on the left. The photo was taken in April 1946 in Hilo, Hawaii. Click the image for a full size photo.)
Most tsunami are caused by earthquakes with epicenters near or on the ocean floor. Not all earthquakes generate tsunami and some tsunami can be small and cause little or no damage. Because of this, people and surfers sometimes go to the beach when there is a tsunami warning. This is not a good idea because it is difficult to know when tsunami will be small and when they will be deadly. It usually takes an earthquake greater than 7.5 on the Richter scale to produce dangerous tsunami. Click here for a tsunami animation (you will need MPEG viewing software).
Sometimes people use the words tidal wave and tsunami to mean the same thing. However, the two are not related. While tsunami refers to dangerous waves caused by underwater disturbances, tidal waves are simply the crest of tides as they travel around the Earth. Tsunami have nothing to do with tides.