lagoon

Concise Oxford English Dictionary © 2008 Oxford University Press:
lagoon/ləˈɡuːn/

▶noun

• 1 a stretch of salt water separated from the sea by a low sandbank or coral reef.

• 2 N. Amer. & Austral./NZ a small freshwater lake near a larger lake or river.

• 3 an artificial pool for the treatment of effluent or to accommodate an overspill from surface drains.

– derivatives
lagoonal adjective.

– origin C17: from Ital. and Sp. laguna, from L. lacuna (see lacuna).