A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. They are well-defined geographically and can be separated from larger bodies of water by sandbars, barrier reefs, coral reefs, or other natural barriers. Lagoons are commonly divided into two types: coastal lagoons and atoll lagoons. Coastal lagoons form along gently sloping coasts and are generally shallower than atoll lagoons. They tend to be separated from the ocean by an island, reef, or sandbank and are connected to the ocean by an inlet. Sea level rise, the amount of existing sediment, and tidal range all contribute to the formation of coastal lagoons. Atoll lagoons form when an island completely subsides beneath the water, leaving a ring of coral that continues to grow upwards. At the center of the ring is a body of water that is often deep. The combination of coral growth and water creates a lagoon.

Lagoons are common coastal features around many parts of the world and are more than just scenic waterbodies; they are important ecosystems that provide habitats for a variety of marine life. 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.

In summary, lagoons are shallow bodies of water separated from larger bodies of water by natural barriers such as reefs, barrier islands, barrier peninsulas, or isthmuses. They are commonly divided into two types: coastal lagoons and atoll lagoons. Coastal lagoons form along gently sloping coasts and are generally shallower than atoll lagoons. They tend to be separated from the ocean by an island, reef, or sandbank and are connected to the ocean by an inlet. Atoll lagoons form when an island completely subsides beneath the water, leaving a ring of coral that continues to grow upwards. Lagoons are important ecosystems that provide habitats for a variety of marine life.