A semi-enclosed body of water which has a free connection to the open sea and within which seawater is measurably diluted by fresh water derived from land drainage.

Sometimes, freshwater from rivers mixes with large freshwater bodies creating a "freshwater estuary" that functions like a typical brackish estuaries.

The five major types of estuaries are coastal plain, bar-built, delta system, tectonic, and fjords.

Estuaries are unique places that are valuable to the environment and to society. An estuary is the thin zone along a coastline (such as bays, lagoons, sounds or sloughs) where freshwater systems and rivers meet, and mix with a salty ocean, becoming brackish.