Impedance is a measure of the opposition to electrical flow in a circuit or device, and it is measured in ohms. In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit. Impedance includes both resistance and reactance, and the resistance component arises from collisions of the current-carrying charged particles with the internal structure of the conductor. The reactance component is an additional opposition to the movement of electric charge that arises from the changing magnetic and electric fields in circuits carrying alternating current.

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. The impedance of a circuit element can be defined as the ratio of the phasor voltage across the element to the phasor current through the element, as determined by the relative amplitudes and phases of the voltage and current.

Impedance is a vector quantity, and it is represented by the symbol Z. The magnitude of the impedance Z of a circuit is equal to the maximum value of the potential difference, or voltage, V (volts) across the circuit, divided by the maximum value of the current I (amperes) through the circuit, or simply Z = V/I. The notion of impedance is useful for performing AC analysis of electrical networks, because it allows relating sinusoidal voltages and currents by a simple linear law.