The thermocline is a thin layer of water in a lake which is sandwiched between the warmer upper layer of water (the epilimnion) and the lower, colder layer of water (hypolimnion).  During the summer months, surface water is heated by the sun which floats over a layer of colder more dense water called the hypolimnion.


The thermocline is the transition layer between the warm water layer at the surface and the cold water layer on the bottom.  The definitions of these layers are based on temperature.   In the thermocline, the temperature decreases rapidly from the mixed layer temperature to the much colder deep water temperature.


The surface layer and the deep water layer are relatively uniform in temperature, while the thermocline represents the transition zone between the two.