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