In the context of fluid mechanics, stratification refers to the spatial variation of density of a given fluid. A fluid is said to be stably stratified if density increases monotonically with depth (dense fluid is never above light fluid) and unstably stratified otherwise. A common feature in stratified water is a pycnocline, which is the location of a sharp transition in density. The figure to the left illustrates a sample stable stratification that uses a vertical tanh profile (blue) and outlines the pynocline in red. Outside of the pycnocline the density is nearly constant, while within the pycnocline the density changes rapidly.
In analogy to pycnocline, a region with rapid temperature variation is called a thermocline and a region with rapid salinity variation is called a halocline.