tidal wave, term properly applied to the crest of a tide as it moves around the earth. The wavelike upstream rush of water caused by the incoming tide in some locations is known as a tidal bore. In popular usage the term tidal wave also is often applied to any destructive wave or to high water not related to tidal phenomena. These latter waves are of two types: tsunamis, which are waves caused by earthquakes, and storm surges (see under storm).