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Impedance is for alternating currents what resistance is for direct currents - a measure of the ability of a circuit to prevent the flow of a current when a voltage is applied across the circuit.

Since both capacitors and inductors influence alternating currents, the impedance takes into account their contribution. Technically, their combined effects are known as reactance; the impedance also includes the effect of any ordinary resistors in the circuit.

Note that the impedance of a capacitor or inductor depends on the frequency of the alternating current applied to it, so the reactance or impedance of a circuit are a function of frequency.