The period when a body of water reaches the same temperature throughout the water mass from the surface to the bottom.

Thermal stratification is a state of a body of water in which the temperature and density of the water differ at different depths. It is divided into direct stratification, where the temperature at the surface is higher than at the bottom (and the density correspondingly lower), and reverse stratification, where the temperature at the surface is lower than at the bottom (and the density higher). Temperature stratification is contrasted with homothermia (from the Greek ὁμός 'equal' + θέρμη 'heat'), a state of a body of water in which water at different depths has the same temperature. In homothermy, the water also has the same density and therefore mixes well.