For DC systems, impedance and resistance are the same, defined as the voltage across an element divided by the current (R = V/I).
In AC systems, the "reactance" enters the equation due to the frequency-dependent contributions of capacitance and inductance. Impedance in an AC system is still measured in ohms and represented by the equation Z = V/I, but V and I are frequency-dependent.
See also: Impedance matching
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