The apparent resistance in an electric circuit to the flow of an alternating current, analogous to the actual electrical resistance to a direct current, being the ratio of electromotive force to the current. It is equal to R2 + X2, where R = ohmic resistance, X = reactance. For an inductive circuit, X = 2pfL, where f = frequency and L = self-inductance; for a circuit with capacity X = 1 / 2pfC, where C = capacity.
A term for the different voltage/current ratios that are used in audio. Terms for impedance are things like 8 ohms, 600 ohms, 10 K ohms, low impedance and high impedance.