An area of relatively shallow water separated from open marine conditions by a barrier is known as what?

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. Lagoons are commonly divided into coastal lagoons and atoll lagoons. 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. 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).

Lagoon is derived from the Italian laguna, which refers to the waters around Venice, the Lagoon of Venice.