Steady-State Error is the difference between the desired input and the actual output of a control system after the transient response has disappeared and the system has reached its steady operating condition. It is commonly expressed as $e_{ss}=\lim_{t\to\infty}e(t)$, and it indicates the accuracy of the system in following a reference input. A smaller steady-state error means better steady-state accuracy, while an error of zero indicates that the output eventually matches the desired input exactly. Steady-state error depends on factors such as system type, controller gain, feedback configuration, and the nature of the input signal (step, ramp, or parabolic).