tidal wave

1. Either of the two swells or crests of surface ocean water created by the gravitational effects of the Moon and Sun and circling the globe on opposite sides to create the daily periods of high and low tides. Also called tidal bulge.

2. An unusual rise in the level of water along a seacoast, as from a storm or a combination of wind and tide. Also called storm surge.

3. A tsunami.

Our Living Language  : The term tidal wave is used in everyday speech to refer to a gigantic and enormously destructive wave caused by an underwater earthquake or volcanic eruption—what scientists would properly call a tsunami. When scientists use the word tidal wave, they normally are referring to an unusually large wave or bulge of water that sometimes occurs around a high tide. These tidal waves are certainly big and powerful, but they are tiny in comparison with tsunamis.