shallow body of water separated from larger body of water by a barrier

A lagoon is a body of salt water partly separated from the sea. There is usually some sort of barrier which separates and protects the lagoon. This may be a pebble or shingle beach, sand bank, or perhaps a coral reef surrounding an atoll. An atoll is an oceanic island formed by a volcano.

Satellite view of Chesil Beach (line in blue running diagonally NW-SE) from Abbotsbury to the Isle of Portland. Between the land and the beach is the Fleet lagoon

Lagoon refers both to coastal lagoons, and the lagoons in atolls, formed by the growth of coral reefs on slowly eroding central islands.