The refers to the region below the mixed layer where salinity changes as you move deeper underwater.

The halocline refers to the region below the mixed layer where the salinity changes as you move deeper underwater.

Explanation:

The ocean has three primary layers. The top layer is the mixed layer which is the warmest due to the exposure to the sun.  The thermocline is the next layer and this layer is in between warm and cool ocean water. This layer is characterized by the rapid change of temperature with depth.

The halocline layer coincides with the thermocline layer and it is the region where salinity rapidly changes with depth.The third and final layer is the deep ocean characterized by higher density and lower temperature compared with the other layers.