We consider a gas inside a closed rigid container completely insulated from the surroundings. If the pressure, volume, temperature, mass and composition of gas do not change with time, then it is in a state of Thermodynamic equilibrium.
A system in the state of Thermodynamic equilibrium, possesses the following equilibrium simultaneously.
(I) Mechanical Equilibrium
There is no unbalanced forces in its interior or between the system and the surroundings.
(II) Thermal Equilibrium
All parts of the system and the surroundings are at the same temperature.
(III) Chemical Equilibrium
The system does not undergo any spontaneous change in its internal structure due to chemical reaction, diffusion etc.