Definition - What does Stratification mean?
Stratification in general terms, is the separation of layers, in this case a liquid, due to the density of varying components in a liquid. Wine for example contains various components of alcohols with different densities. This is why, for the aging of wine, it is important to stir wines or rack them from time to time.
WineFrog explains Stratification
Stratification is a topic which is addressed in the processing of wine from fermentation to its elaboration and aging.
In the case of fermentation, grape juice is held in a fermentation tank, with a cooling or temperature-controlled jacket around its center. Without pump-overs or punch-downs, fermenting wine will tend to stratify. This means that the varying components which are created in fermentation can separate in the vat, like oil and water. This is due to the varying densities of liquids which form during fermentation. The cap will float on the top of the vat while, then the juice which has been turned into wine will rest under the cap, with the remaining grape juice below. This is why pump-overs and punch-downs are necessary during fermentation so that the juice is evenly fermented and completed.
For the elaboration of wines, like white wines, they are kept at low temperatures. However, the cooling jacket is not wrapped around the entire tank. Therefore different alcohols present in the wine such as ethanol and glycerol will be separated in defined layers. This is stratification. This is also the reason that white wine needs to be stirred during aging.