Definition of thermodynamic equilibrium
When the system is in equilibrium, its macro-physical properties do not change with time, but from a micro perspective, the particles that make up the system are in constant thermal motion. The equilibrium in thermodynamics is a dynamic equilibrium, so it is called "thermodynamic equilibrium". For example, if two copper blocks at different temperatures are put together, heat transfer will occur until the two objects are at the same temperature, and the two objects are in thermal equilibrium. The saturated vapor in contact with the liquid is in equilibrium. From a macro perspective, pressure, density, and temperature are unchanged. In this case, the molecules are still changing from liquid to gaseous, while the molecules are returning from vapor to liquid at the same rate. Therefore this type of balance is called thermodynamic balance.