Any body of salt water or brackish water that is a shallow separation from the rest of the sea is considered a lagoon, and they form when sand banks or reefs build up along shallow waters near the coast or when coral reefs grow on central islands that are sinking. Estuaries, or fresh water streams, feed the lagoons as they grow.
The word "lagoon" dates back to 1769, and its first use refers to the stretch of salt water dotted with islands along the edge of Venice, which has the Lido's barrier beaches protecting it from the stormy surges from the Adriatic Sea. The Italian word "laguna," which is based on the Latin word "lacuna," or empty space, was the inspiration for this English term. Some lagoons have taken on other names, such as North Carolina's Albemarle Sounds or the Banana River in Florida, but they are still lagoons.
When a lagoon forms as part of the ecosystem of a coral reef, the term means the same thing as "back reef," which is the technical term that scientists use. Whether the lagoon is part of a coral system or not, its shallow waters make it extremely sensitive to changes in the environment.