In the context of diffusion, equilibrium refers to the state where the concentration of molecules is uniform throughout a given space. When diffusion occurs, molecules move from areas of higher concentration to areas of lower concentration, attempting to achieve balance.
At equilibrium, the movement of molecules continues, but there is no net change in concentration; the rates of diffusion into an area and out of that area are equal. This means that even though individual molecules may still be moving, the overall distribution of molecules does not change. This concept is crucial in understanding processes in biology and chemistry, where diffusion is a key mechanism for mass transport.