An estuary is semi-enclosed coastal body of water with a one or more rivers or streams flowing into it, and with a free connection to the open sea. The key feature of an estuary is that it is a mixing place for sea water and fresh water to supply fresh water. A tide is a necessary force to maintain a dynamic relationship at the meeting between the two waters. In non-tidal seas, rivers naturally form deltas or liman.

An estuary is typically the tidal mouth of a river, and estuaries are often characterised by sedimentation or silt carried in from terrestrial runoff. Estuaries are more likely to occur on submerged coasts, where the sea level has risen in relation to the land, this process floods valleys to form rias and fjords. These can become estuaries if there is a stream or river flowing into them. Large estuaries, like Chesapeake Bay and Puget Sound often have many streams flowing into them and can have complex shapes. Estuaries are often given names like bay, sound, fjord, etc. The terms are not mutually exclusive.

The Estuary is also a term used to describe the region of the River Thames and adjoining communities east of Central London in the United Kingdom. The term Estuary English is frequently used to describe the accent of the people in this region, though it has since spread across South East England. It is sometimes mistaken for the Cockney accent by listeners unfamiliar with the varieties.