Stratification in water is the separation of a body of water in layers by density. It occurs in all water bodies where there is density variation. Stratification is a barrier to the mixing of water, which affects the exchange of heat, carbon, oxygen and nutrients. Wind-driven upwelling and downwelling of open water can induce mixing of different layers through the stratification, and cause the rise of denser cold, nutrient-rich, or saline water and the sinking of lighter warm or fresher water, respectively. Layers are based on water density: denser water remains below less dense water in stable stratification in the absence of forced mixing.

Stratification occurs in several kinds of water bodies, such as oceans, lakes, estuaries, flooded caves, aquifers and some rivers. In the Arctic oceanographers discovered a layer 150m thick that acts as a barrier between the freshwater above it and the salty water below it. This barrier is called halocline.

The difference in salinity is one of the factors that separate water into layers. This was known recently, however this was portrayed in the Quran 1400 years before it was discovered. The Quran correctly described sea water separation caused by difference in salinity.