A lagoon is a shallow body of water separated from a larger body of water by reefs, barrier islands, or a barrier peninsula. Lagoons are commonly divided into coastal lagoons and atoll lagoons. They have also been identified as occurring on mixed-sand and gravel coastlines. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as estuaries. Lagoons are common coastal features around many parts of the world.
Lagoons are shallow, often elongated bodies of water separated from a larger body of water by a shallow or exposed shoal, coral reef, or similar feature. Some authorities include fresh water bodies in the definition of "lagoon", while others explicitly restrict "lagoon" to bodies of water with some degree of salinity. The distinction between "lagoon" and "estuary" also varies between authorities. Richard A. Davis Jr. restricts "lagoon" to bodies of water with little or no fresh water inflow, and little or no tidal flow, and calls any bay that receives a regular flow of fresh water an "estuary". Davis does state that the terms "lagoon" and "estuary" are "often loosely applied, even in scientific literature".  Timothy M. Kusky characterizes lagoons as normally being elongated parallel to the coast, while estuaries are usually drowned river valleys, elongated perpendicular to the coast.      Coastal lagoons are classified as inland bodies of water.  
When used within the context of a distinctive portion of coral reef ecosystems, the term "lagoon" is synonymous with the term "back reef" or "backreef", which is more commonly used by coral reef scientists to refer to the same area. 
Many lagoons do not include "lagoon" in their common names. Currituck, Albemarle and Pamlico sounds in North Carolina,  Great South Bay between Long Island and the barrier beaches of Fire Island in New York,  Isle of Wight Bay, which separates Ocean City, Maryland from the rest of Worcester County, Maryland,  Banana River in Florida,  Lake Illawarra in New South Wales,  Montrose Basin in Scotland,  and Broad Water in Wales have all been classified as lagoons, despite their names. In England, The Fleet at Chesil Beach has also been described as a lagoon.
In some languages the word for a lagoon is simply a type of lake: In Turkish, a lake is göl, a lagoon is gölcük. Similarly, in Chinese a lake is hu (湖), a lagoon is xihu. (潟湖)
Contrariwise, several other languages have specific words for such bodies of water. In Spanish, coastal lagoons generically are laguna costera, but those on the Mediterranean coast are specifically called albufera ( es): In Russian and Ukrainian those on the Black Sea are liman (лиман), while the generic word is laguna (Лагуна). Similarly, in the Baltic, Danish has the specific Nor ( da), and German the specifics Bodden and Haff ( de), as well as generic terms derived from laguna. In New Zealand the Māori word hapua refers to a coastal lagoon formed at the mouth of a braided river where there is mixed sand and gravel beaches, while the word waituna is the more general term.
Some languages differentiate between coastal and atoll lagoons: In French, lagon( fr) refers specifically to an atoll lagoon, while coastal lagoons are described as étang( fr), the generic word for a still lake or pond. In Vietnamese, Đầm san hô refers to an atoll lagoon, whilst Đầm phá is coastal.
In Latin America, the term laguna in Spanish, which lagoon translates to, may be used for a small fresh water lake in a similar way a creek is considered a small river. However, sometimes it is popularly used to describe a full-sized lake, such as Laguna Catemaco in Mexico, which is actually the third largest lake by area in the country. The brackish water lagoon may be thus explicitly identified as a "coastal lagoon" (laguna costera). In Portuguese, a similar usage is found: lagoa may be a body of shallow sea water, or a small freshwater lake not linked to the sea.
Lagoon is derived from the Italian laguna, which refers to the waters around Venice, the Lagoon of Venice. Laguna is attested in English by at least 1612, and had been Anglicized to "lagune" by 1673. In 1697 William Dampier referred to a "Lagune or Lake of Salt water" on the coast of Mexico. Captain James Cook described an island "of Oval form with a Lagoon in the middle" in 1769. 
Atoll lagoons form as coral reefs grow upwards while the islands that the reefs surround subside, until eventually only the reefs remain above sea level. Unlike the lagoons that form shoreward of fringing reefs, atoll lagoons often contain some deep (>20 metre; 65') portions.
Coastal lagoons form along gently sloping coasts where barrier islands or reefs can develop off-shore, and the sea-level is rising relative to the land along the shore (either because of an intrinsic rise in sea-level, or subsidence of the land along the coast). Coastal lagoons do not form along steep or rocky coasts, or if the range of tides is more than 4 metres (13 ft). Due to the gentle slope of the coast, coastal lagoons are shallow. A relative drop in sea level may leave a lagoon largely dry, while a rise in sea level may let the sea breach or destroy barrier islands, and leave reefs too deep under water to protect the lagoon. Coastal lagoons are young and dynamic, and may be short-lived in geological terms. Coastal lagoons are common, occurring along nearly 15 percent of the world's shorelines. In the United States, lagoons are found along more than 75 percent of the Eastern and Gulf coasts.  
Coastal lagoons are usually connected to the open ocean by inlets between barrier islands. The number and size of the inlets, precipitation, evaporation, and inflow of fresh water all affect the nature of the lagoon. Lagoons with little or no interchange with the open ocean, little or no inflow of fresh water, and high evaporation rates, such as Lake St. Lucia, in South Africa, may become highly saline. Lagoons with no connection to the open ocean and significant inflow of fresh water, such as the Lake Worth Lagoon in Florida in the middle of the 19th century, may be entirely fresh. On the other hand, lagoons with many wide inlets, such as the Wadden Sea, have strong tidal currents and mixing. Coastal lagoons tend to accumulate sediments from inflowing rivers, from runoff from the shores of the lagoon, and from sediment carried into the lagoon through inlets by the tide. Large quantities of sediment may be occasionally be deposited in a lagoon when storm waves overwash barrier islands. Mangroves and marsh plants can facilitate the accumulation of sediment in a lagoon. Benthic organisms may stabilize or destabilize sediments.  
- Davis, Richard A., Jr. (1994). The Evolving Coast. New York: Scientific American Library. pp. 101, 107. ISBN 9780716750420.
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