The zone between waters with different densities. An example from an estuary would be a pycnocline separating deep, more saline water and shallow, more fresh water.

the vertical zone in the water column where density changes most rapidly with depth

the region of the water column characterized by the strongest change in density with depth. Temperature falls and salinity increases in this zone.

a layer of rapid change in water density with depth

a separation between two layers of different densities

Boundary between colder, saltier, denser water at the bottom and warmer, less salty, less dense water on the surface

The natural stratification of the Sound's waters that occurs during the summer when warmer, fresher water floats on the cooler, saltier water that is more dense. The density difference between these two layers prevents the mixing of the well-oxygenated surface waters with the bottom waters. – website

The pycnocline is the depth in the water column where there is an abrupt change in density, temperature, and salinity. A pycnocline often forms in the Chesapeake Bay and its tidal tributaries when the lighter, warmer, and fresher water coming downstream from the spring rains overlays the denser, colder, and saltier water of the salt wedge bringing water upstream from the ocean.

A pycnocline is a layer across which there is a rapid change in water density with depth. In freshwater environments such as lakes this density change is primarily caused by water temperature, while in seawater environments such as oceans the density change may be caused by changes in water temperature and/or salinity.