what is the difference between thermal decomposition and thermal dissociation?

Can I please get it in simple, decipherable and understandable words?

Thermal decomposition:-

Reaction in which heat is used  for the breaking of a chemical substances in two or more parts.

CaCO₃+heat --> CO₂+CaO

Thermal dissociation:-

A compound is breaks reversibly into two parts in a single step

NH₄Cl+heat --> NH₃+HCl