1. The oscillation of a body of water at its natural period. Coastal measurements of sea level often show seiches with amplitudes of a few centimeters and periods of a few minutes due to oscillations of the local harbor, estuary, or bay, superimposed on the normal tidal changes. 2. In the Great Lakes area of the United States, any sudden rise in the water of a harbor or a lake, whether or not it is oscillatory. Although inaccurate in a strict sense, this usage is well established in the Great Lakes area.