The layer of ocean water where temperature changes rapidly with depth is called the thermocline.
Ocean water near the surface is stirred by wind and waves, creating a relatively uniform mixed layer. Beneath it, temperature can fall sharply over several hundred metres. This steep temperature gradient forms the thermocline, which separates warmer surface water from colder, denser deep water.
Thermoclines are strongest in tropical and temperate oceans and may be weak or absent in polar regions, where surface water is already very cold. Their depth also changes with season: winter mixing can push a thermocline deeper, while summer heating often makes it shallower.
A common mix-up is the pycnocline, which describes a rapid change in water density, and the halocline, which describes a rapid change in salinity. Temperature, salinity and density often change together, but the thermocline specifically refers to temperature.