lagoon

A shallow body of sea water separated from the open sea by island barriers. Coastal lagoons occur in regions where little surface run-off enters the sea. Organic reefs may also form lagoons, as is the case at coral atolls and barrier reefs.

A lagoon is a body of comparatively shallow salt water separated from the deeper sea by a shallow or exposed sandbank, coral reef, or similar feature. Thus, the enclosed body of water behind a barrier reef or barrier islands or enclosed by an atoll reef is called a lagoon. Lagoon refers to both coastal lagoons formed by the build-up of sandbanks or reefs along shallow coastal waters, and the lagoons in atolls, formed by the growth of coral reefs on slowly sinking central islands. Once a lagoon barrier has formed, finer sediments can settle out in the relatively quiet water behind the barrier, including sediments brought into the lagoon by rivers. As a result, water conditions in the lagoon can differ significantly from the open water of the sea in temperature, salinity, dissolved oxygen and sediment load.

In many English-speaking countries, coastal lagoons are sometimes called sounds, bays, rivers, or lakes. Albemarle Sound in North Carolina, Great South Bay, between Long Island and the barrier beaches of Fire Island in New York, Banana River in Florida and Lake Illawarra in New South Wales are all lagoons. In the UK there are lagoons at Montrose, (Scotland) and Tywyn, (Wales), whilst the expanse of water inside Chesil Beach, England, known as the fleet, could also be described as a lagoon.

In Mexico often the use of "laguna", which lagoon translates to, is used to describe a lake, such as Laguna Catemaco.