speaking, a system
is termed stable if no rearrangement of its parts can form a system of lower free energy.
In practice, the term is used with an implicit proviso regarding the
transformations to be considered. Hydrogen is not considered unstable merely because it is subject to nuclear
fusion at extreme temperatures.
A system is usually regarded as stable (more precisely,
as kinetically stable) if its rate of transformation to a state of lower free energy
is negligible (by some standard) under the ambient conditions. In nanomechanical systems,
a structure can commonly be regarded as stable if it has an extremely low
rate of transformations when subjected to its intended operating conditions.