What is the definition of thermodynamic stability?
The full phrase should be thermodynamic stability with respect to ____, where the dash indicates a process, or a chemical reaction.
A mixture of hydrogen and oxygen is thermodynamically unstable with respect to water formation.
Similarly, a diamond is not forever (which may not please De Beers and ladies). It is thermodynamically unstable with respect to conversion to graphite.
Also, thermodynamic stability is a relative term which is often contrasted with reactivity or kinetic stability.
Diamond is kinetically stable at room temperature for the same process (lucky ladies can smile again).