Impedance

• The ratio of voltage to current in an AC circuit.

• The combination of fixed DC resistance and frequency dependent AC reactance.

• Impedance possesses both a magnitude, expressed in ohms, and a phase angle specified in degrees or radians.
  eg. Z = 16 ohms +27 degrees

• Impedance (magnitude) and phase angle both vary with frequency

• At 0 degrees phase angle the impedance is equal to the resistance of the circuit