Final answer:
A lagoon is a shallow body of water separated from larger bodies by sand spits, barrier islands, or coral reefs, typically exhibiting low energy, high silt and clay content, and variable salinity.
Step-by-step explanation:
A body of water cut off from a larger body of water by a reef of sand, such as the examples found in Mongolia, is defined as a lagoon. Lagoons are typically shallow and can be separated from a larger body of water, like an ocean or sea, by a natural barrier. This barrier could be in the form of a sand spit, a barrier island, or even a coral reef, allowing some water exchange but maintaining an area of low energy, protected from direct marine conditions.
Lagoons often contain silt and clay and the salinity can vary greatly, sometimes being much higher than that of the nearby sea. They are connected to the larger body of water by one or more narrow straits and may experience significant changes in water level due to tides. An example of this type of formation can be seen in East Matagorda Bay, Texas.