Impedance See Also: Ohm's Law Measure of the degree to which an Electric Circuit resists electric - Current flow when a Voltage (see Potential) is impressed across its terminal. Impedance, expressed in Ohms, is the ratio of the voltage impressed across a pair of terminals to the current flow between those terminals. In Direct Current (DC) circuits, impedance corresponds to Resistance. In Alternating Current (AC) circuits, impedance is a function of resistance, Inductance, and Capacitance. Inductors and Capacitors build up voltages that oppose the flow of current. This opposition, called Reactance, must be combined with resistance to find the impedance. The reactance produced by inductance is proportional to the Frequency of the alternating current, whereas the reactance produced by capacitance is inversely proportional to the frequency. In order to transfer maximum electrical power from one device to another, the two impedances must be matched.