A lagoon is a body of comparatively shallow salt water separated from the deeper sea by a shallow or exposed sandbank, coral reef, or similar feature. Thus, the enclosed body of water behind a barrier reef or enclosed by an atoll 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 (see Venetian Lagoon).

On the Mid-Atlantic Coast of the United States, such lagoons enclosed by barrier islands retain the more traditional designation sounds. 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.

On the Pacific side of the North American continent, particularly in northern California, there occur scattered, protected and picturesque lagoons.

Geographical lagoons

• Laguna de Lobos, Buenos Aires Province, Argentina
• Buada Lagoon , Buada, Buada District , Nauru

lagoon is also given to an artificial pond used for treatment of effluent (usually to oxygenate or settle particulates) or to accommodate an overspill from surface drains during heavy rainfall.