1. In geophysics, a layer in a body of water at which the rate of temperature decreases when depth is at a maximum; such as, a thin layer of water at the depth range of 300-800 meters in the tropical and subtropical basins, between the deep abyssal waters and the surface mixed layer.
2. A horizonal temphorizontal temperature discontinuity layer in a lake in which the temperature falls by at least 1°C per meter depth.
3. A boundary layer in the sea in which temperature changes sharply with depth.