Thermocline is the layer in between 'Mixed Layer' and 'Deep Water Layer' in oceans, lakes etc or even in our atmosphere.
Now split that word- thermo: meaning heat and 'cline': derived from decline. This is a transition layer in the middle where the temperature of the water declines with the increase in depth.
The 'mixed layer' being closer to the surface has a temperature of 30-35° C and extend till 300-400m.
Within 'thermocline layer' the temperature drastically falls from 30-35° C to 4°C.
Below 'thermocline layer', we get 'deep water layer' where the temperature of water stays at 4°C.