In thermodynamics, what exactly is equilibrium? Does that mean everything inside a system has equal temperature? In case like fluid flow between two parallel plates of different temperatures, energy transfer from the plate to the fluid, and different point has different temperature, is the system not in equilibrium? Is temperature defined only for equilibrium system? And if so, how can we talk about non-uniform temperature? Can someone clarify this, and give me some examples of equilibrium and non-equilibrium systems? Thanks.