What is Mixed layer, thermocline, halocline, pycnocline?

The mixed layer is the surface layer of the ocean whose temperature, salinity and density are nearly uniform with depth; its depth is read on a profile as the depth where the temperature first differs from its surface value by a chosen threshold, often $0.5\,{}^{\circ}\mathrm{C}$. The thermocline, the halocline and the pycnocline are the layers below it in which the temperature, the salinity and the density respectively change fastest with depth.