What definition of a thermocline? what causes this phenomenon?

Well what causes this is a steep temperature gradient in a body of water such as a lake, marked by a layer above and below which the water is at different temperatures.what cause this is - A thermocline (sometimes metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, such as an ocean or lake, or air, such as an atmosphere) in which temperature changes more rapidly with depth than it does in the layers above or below