Impedance is a measure of how much a circuit resists the flow of electric current. It combines both resistance, which opposes current flow, and reactance, which arises from capacitors and inductors that store and release energy. Impedance is expressed in ohms and is crucial in understanding how electrical components interact in circuits. In alternating current (AC) systems, impedance affects how voltage and current behave. It determines the phase difference between them, influencing the efficiency of power transfer. Engineers use impedance to design circuits in various applications, including audio equipment and telecommunications, ensuring optimal performance and signal integrity.