Tsunami (pron: 'soo-nar-me') is a Japanese word; 'tsu' meaning harbour and 'nami' meaning wave. The phenomenon is usually associated with earthquakes, landslides or volcanic eruptions in, or adjacent to oceans, and results in sudden movement of the water column. Until recently tsunami were called tidal waves, even though the event has nothing to do with tides.
A tsunami is different from a wind generated surface wave on the ocean. The passage of a tsunami involves the movement of water from the surface to the seafloor which means its speed is controlled by water depth. Consequently, as the wave approaches land and reaches increasingly shallow water it slows. However, the water column still in deeper water is moving slightly faster and catches up, resulting in the wave bunching up and becoming much higher. A tsunami is often a series of waves and the first may not necessarily be the largest.
When a tsunami travels over a long and gradual slope, it allows time for the tsunami to grow in wave height. This is called shoaling and typically occurs in shallow water less than 100m. Successive peaks can be anywhere from five to 90 minutes apart. In the open ocean, even the largest tsunami are relatively small with wave heights of less than one metre. The shoaling effect can increase this wave height to a degree such that the tsunami could potentially reach an onshore height of up to 30 metres above sea level. However, depending on the nature of the tsunami and the nearshore surroundings, the tsunami may create only barely noticeable ripples.
Interesting fact: Tsunami can travel at speeds up to 950km/h in deep water which can be represented by the speed of a passenger jet.
Source: Geoscience Australia.