- 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.
- 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.
