1. The total opposition to the flow of alternating current in a circuit because of resistance and reactance (a form of electric resistance observed in an alternating current).
2. The opposition which a circuit presents to electric current.

The impedance includes both resistance and reactance.

Resistance results from collisions of the current-carrying charged particles with the internal structure of the conductor while reactance is an additional opposition to the movement of an electric charge that comes from the changing electric and magnetic fields in circuits that carry alternating current.

A device that measures the complex ratio of voltage to current in a given circuit at a given frequency.