steady-state approximation
Imagine a bathtub where water pours in from the tap and drains out the plughole at the same time. After a short while the water level holds steady — not because nothing is flowing, but because the inflow and outflow match. You do not need to track every drop; you can just say the level stays roughly constant. The steady-state approximation applies this same idea to a fleeting reaction intermediate.
More precisely, the steady-state approximation is a method for simplifying the maths of a multi-step mechanism by assuming that a highly reactive intermediate is consumed almost as fast as it is formed, so its concentration stays nearly constant and low throughout most of the reaction. Setting the rate of formation of that intermediate equal to its rate of consumption gives an equation you can solve for its tiny concentration, which then drops out of the algebra, yielding the overall rate law in terms of the things you can actually measure.
Why it matters: this trick is one of the main tools for turning a proposed mechanism into a predicted rate law, letting chemists test whether a mechanism agrees with experiment — it is, for instance, how the Michaelis–Menten equation is derived. The honest caveat is that the approximation only holds when the intermediate really is short-lived and stays at low concentration; early in the reaction, before steady state is reached, and for stable intermediates, it can fail.
A mechanism makes a reactive intermediate in one step and uses it up in two later steps. Rather than solve the full tangle of equations, a chemist sets 'rate made = rate used' for that intermediate. This pins its tiny concentration, which then cancels out, leaving a clean rate law that can be compared straight to the lab data.
Set an intermediate's rate of formation equal to its rate of consumption, then solve.
Steady state does not mean the intermediate's amount is zero or unchanging forever — only that it is small and roughly constant during the bulk of the reaction. It is a close cousin of, but not identical to, the rate-determining-step shortcut.