impedance (symbol Z) Property of a component in an electric circuit that opposes the passage of current. In a direct current (DC) circuit, the impedance corresponds to the resistance (R). In an alternating current (AC) circuit with capacitance or inductance, the additional property of reactance (X) has to be allowed for, as expressed in the equation Z² = R² + X². All of these quantities are measured in ohms.

im·ped·ance / imˈpēdns/ • n. the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.