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A term used in electronics, audio and electrical circuitry. Impedance is the general term for the ratio of voltage to current; it is measured in Ohms and is affected by both inductance and reactance in a circuit. It is similar to electrical resistance, in that it is a measure of the opposition to the flow of electricity. Impedance is meaningful only for alternating current (AC) circuits, and as a factor, changes value as the frequency of the electricity changes.