Impedance is a measure of how much a circuit resists the flow of alternating current (AC) and is represented as a complex quantity combining resistance and reactance. It is expressed in ohms and takes into account both the resistance (real part) and the reactance (imaginary part) due to inductors and capacitors. Impedance is crucial in AC circuit analysis, influencing how voltage and current relate in the circuit. In summary, it represents the total opposition to AC flow, affecting the performance of electronic circuits.