Electrical impedance (or simply impedance or total resistance) measures opposition to a sinusoidal alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits. Impedance is the complex generalization of resistance. In most cases, impedance is a function of the frequency (ω in radians/sec or in hertz), resistance (in ohms), inductance (in henries), and capacitance (in farads):

The reactance is the imaginary part of the impedance, ; . We consider the impedance to be a function of frequency only; and are constant parameters. The impedance over a range of excitation frequencies is plotted. In the case of biological tissues, the impedance is .

The plots show the impedance (total resistance) in red, the biological tissue impedance in gray, and the capacitance resistance in green.