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.
Lagoon refers both to coastal lagoons, and the lagoons in atolls, formed by the growth of coral reefs on slowly eroding central islands.
Chesil Beach, the Fleet and the Isle of Portland, from the north-west over Abbotsbury
Blaketown Lagoon, New Zealand: it is connected to the sea.
Fangataufa is an atoll in the Pacific Ocean. Its lagoon is about 5km by 8 km. The whole atoll is about 9.5 km by 9.5km, with a landmass of about 5 km2