rhythmic oscillation of water in a lake or a partially enclosed coastal inlet, such as a bay, gulf, or harbour. A seiche may last from a few minutes to several hours or for as long as two days. The phenomenon was first observed and studied in Lake Geneva (Lac Léman), Switzerland, in the 18th century.

Seiches may be induced by local changes in atmospheric pressure. They also may be initiated by the motions of earthquakes and by tsunamis, in the case of coastal inlets. Seismic surface waves from the Alaska earthquake of 1964, for example, triggered seiche surges in Texas in the southwestern United States when they passed through the area. Studies of seiche behaviour have shown that once the surface of the water is disturbed, gravity seeks to restore the horizontal surface, and simple vertical harmonic motion ensues. The impulse travels the length of the basin at a velocity dependent on the water depth and is reflected back from the basin's end, generating interference. Repeated reflections generate standing waves with one or more nodes, or points, that experience no vertical motion. The length of the lake is an exact multiple of the distance between nodes.

Seiches may disturb shipping by generating strong reversible currents at the entrances to harbours or by causing moored vessels to oscillate against their mooring cables and break free. Seiches also may drown unwarned persons on piers and shores.