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 alternating current (AC) circuits, where it affects how voltage and current interact. In practical terms, understanding impedance helps in designing electrical systems, such as audio equipment and radio transmitters. It ensures that components like speakers and amplifiers work efficiently together, optimizing performance and preventing damage. Proper impedance matching is essential for effective signal transmission in various applications.