Thermal stability is the stability of a molecule at high temperatures; i.e. a molecule with more stability has more resistance to decomposition at high temperatures.
Thermal stability also describes, as defined by Schmidt (1928), the stability of a water body and its resistance to mixing. This is the amount of work needed to transform the water body (e.g. a lake) to a uniform water density. The Schmidt stability 'S' is commonly measured in Joule per square meter or g*cm/cm. Compare Stratification.