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

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.