What is impedance?

Impedance (Z) represents the total opposition to alternating current (AC) in a circuit, combining both pure resistance and reactance into a single measurement. Just as Ohm's Law states that resistance is voltage divided by current (R = E / I) in DC circuits, the exact same mathematical relationship holds for AC circuits. Therefore, impedance is defined as the ratio of voltage (E) to current (I). The distractors represent entirely different electrical concepts. For instance, the product of voltage and current defines electrical power (Watts), while the inverse ratio of current to voltage defines admittance (Siemens), which is the reciprocal of impedance.