• Equilibrium means a condition of balance.
•  In thermodynamics the concept includes not only a balance of forces, but also a balance of other influences. 
• Each kind of influence refers to a particular aspect of thermodynamic (complete) equilibrium. 
• Thermal equilibrium refers to an equality of temperature, mechanical equilibrium to an equality of pressure, and phase equilibrium to an equality of chemical potentials 
• Chemical equilibrium is also established in terms of chemical potentials. For complete equilibrium the several types of equilibrium must exist individually.
• To determine if a system is in thermodynamic equilibrium, one may think of testing it as follows:Isolate the system from its surroundings and watch for changes in its observable properties. If there are no changes, it may be concluded that the system was in equilibrium at the moment it was isolated. The system can be said to be at an equilibrium state. When a system is isolated, it cannot interact with its surroundings; however, its state can change as a consequence of spontaneous events occurring internally as its intensive properties, such as temperature and pressure, tend toward uniform values. When all such changes cease, the system is in equilibrium. At equilibrium. temperature and pressure are uniform throughout. If gravity is significant, a pressure variation with height can exist, as in a vertical column of liquid.