Professional Answer: Surge Impedance Loading (SIL) is the amount of power a transmission line delivers to a purely resistive load that is equal to the line's surge impedance. When a line is loaded at its SIL, the reactive power generated by its shunt capacitance is perfectly balanced by the reactive power consumed by its series inductance. This results in a flat voltage profile along the entire length of the line, meaning the voltage at the receiving end is equal to the voltage at the sending end. Understanding SIL is vital for system stability and for optimizing the loading capacity of long transmission lines.