The Wash is a shallow natural rectangular bay and multiple estuary on the east coast of England, United Kingdom. The bay is an inlet of the North Sea. It is the largest estuary system in the United Kingdom, the largest natural bay in England and one of the most important places of conservation in Europe, with several nature reserves.

An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.