epilimnion
noun
epi·lim·ni·on ˌe-pə-ˈlim-nē-ˌän 
-nē-ən
: the water layer overlying the thermocline of a lake
Word History
Etymology
epi- + Greek límnē "standing water, pool, marshy lake" + -ion, noun suffix — more at limnetic
Note: See note at hypolimnion.
First Known Use
circa 1910, in the meaning defined above