An approximation used to simplify the analysis of kinetics, in which the reactants (R) go to products (P) via intermediates (I). The steady-state approximation is that the rate of change of the concentration of I with time (t) is zero, i.e. the concentration of I is constant with time. This is a good approximation if the rate of the reaction from I to P is much greater than the rate of the reaction from R to I. It is important to emphasize that the steady-state approximation is not the same as chemical equilibrium.