Equilibrium is a state of balance or stability in a system. In the context of solutes, it refers to a situation where the concentration of solutes is equal on both sides of a membrane or barrier. This means that there is no net movement of solutes from one side to the other, as the concentration gradient is zero. Equilibrium can be reached through various processes, such as diffusion, osmosis, or active transport, depending on the nature of the solutes and the conditions of the system.