Lagoon is a backwater or brackish water lake or a salt water lake formed by the waves. They have a narrow link with the ocean water.

Formation of a lagoon When the waves recede, the eroded materials may be deposited perpendicular to the coast forming a tongue like feature known as sand spit.

Due to the change in the direction of the wind, waves change their direction and the deposits may be deposited at right angles to the sand spit and parallel to the coast forming a sand bar.

The sand bar sometimes gets closer to the sea shore, enclosing a part of the saline water of the ocean. These are known as lagoons. Lagoons are commonly formed off the coast of Kerela, e.g. Velli, Vemband on the west coast. On the east coast there is the Chilka Lake (Orissa), Pulicat Lake (Madras, Tamil Nadu).