A seiche ( SAYSH) is a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, swimming pools, bays, harbors, caves and seas. The key requirement for formation of a seiche is that the body of water be at least partially bounded, allowing the formation of the standing wave.

The term was promoted by the Swiss hydrologist François-Alphonse Forel in 1890, who was the first to make scientific observations of the effect in Lake Geneva. The word had apparently long been used in the region to describe oscillations in alpine lakes. According to Wilson (1972), this Swiss French dialect word comes from the Latin word siccus meaning "dry", i.e., as the water recedes, the beach dries. The French word sec or sèche (dry) descends from the Latin.

Seiches in harbours can be caused by long-period or infragravity waves, which are due to subharmonic nonlinear wave interaction with the wind waves, having periods longer than the accompanying wind-generated waves.