metalimnion
/ˌmɛtəˈlɪmnɪən/ /ˌmɛtəˈlɪmnɪɒn/Chiefly Limnology
noun
The layer of water in a stratified lake which lies beneath the epilimnion and above the hypolimnion, in which the temperature decreases rapidly with depth.
Origin
1930s. From meta- + -limnion, after epilimnion and hypolimnion.