A lagoon is a shallow body of water separated from a larger body of water, usually the ocean, by a barrier like a sandbar, coral reef, or narrow strip of land. Picture a calm pool of seawater nestled behind a protective ridge of sand or rock, with maybe just a small opening connecting it to the rougher ocean beyond.

Lagoons are special places where the water stays calmer than the open sea. The barrier breaks up the waves before they reach the lagoon, creating a peaceful environment where different creatures can thrive. Many young fish grow up in lagoons because the sheltered water protects them from predators and strong currents. Coral atolls in the South Pacific often have beautiful blue lagoons in their centers, ringed by coral reefs.

The word can also describe similar enclosed pools in other settings. Some coastal towns have lagoons that form naturally between the shore and offshore sandbars. Venice, Italy, is famously built on islands within a lagoon. Even some sewage treatment plants use artificial lagoons to help purify water naturally before releasing it.

The key feature of any lagoon is that separation: it's connected to but protected from the larger body of water nearby, creating its own distinct, usually calmer environment.