Impedance is a concept in electrical engineering that measures the opposition of an electrical circuit to the flow of alternating current (AC). It is represented by the symbol "Z" and is analogous to resistance in a direct current (DC) circuit. However, unlike resistance, which only considers the real component of the circuit's opposition to current, impedance takes into account both the magnitude and phase relationship between the voltage and current. Impedance is measured in ohms (Ω) and is a complex quantity, consisting of a real part (resistance) and an imaginary part (reactance).