A lagoon is a stretch of comparatively shallow salt water separated from the deeper sea by a shallow or exposed sandbank, coral reef, or similar structure. Thus, the enclosed body of water behind a barrier reef such as an atoll's reef is called a lagoon.
This application of lagoon in English dates from 1769. It adapted and extended the sense of the Venetian laguna (cf Latin lacuna, 'empty space'), which specifically referred to Venice's shallow, island-studded stretch of salt water, protected from the Adriatic by the barrier beaches of the Lido.
On the Mid-Atlantic Coast of the United States, such lagoons enclosed by barrier islands retain the more traditional designation 'soundss.' Albemarle Sound and Pamlico Sound might more accurately be termed lagoons. Though Long Island Sound is a true sound, Great South Bay, between Long Island and the barrier beaches of Fire Island, is actually a lagoon.