Impedance

Electrical impedance, measure of the total opposition that a circuit or a part of a circuit presents to electric current. Impedance includes both resistance and reactance (qq.v.). The resistance component arises from collisions of the current-carrying charged particles with the internal structure of the conductor.

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.

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. 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.