Impedance (measured in Ohms) is the measure of the resistance to current that a circuit presents over a given voltage, or the ratio between the rms values of current and voltage. In DC circuits, impedance is the same as resistance.

Impedance is a complex number, with resistance being the real part and reactance being the imaginary part. In general, the formula for impedance is

$ \ Z = R + jX = R + j (X_L - X_C) $

With R being the resistance, j the imaginary unit (not i to avoid confusion with current), XL being the inductive reactance, and XC being the capacitive reactance.