http://en.wikipedia.org/wiki/Electrical_impedance
Wikipedia
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that a circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current (AC) circuit. Impedance extends the concept of resistance to AC circuits, and possesses both magnitude and phase, unlike resistance which has only magnitude. When a circuit is driven with direct current (DC), there is no distinction between impedance and resistance
Impedance is defined as the frequency domain ratio of the voltage to the current. In other words, it is the voltage–current ratio for a single complex exponential at a particular frequency w. In general, impedance will be a complex number, with the same units as resistance, for which the SI unit is the ohm (O). For a sinusoidal current or voltage input, the polar form of the complex impedance relates the amplitude and phase of the voltage and current. In particular,
The magnitude of the complex impedance is the ratio of the voltage amplitude to the current amplitude.
The phase of the complex impedance is the phase shift by which the current is ahead of the voltage.
The reciprocal of impedance is admittance (i.e., admittance is the current-to-voltage ratio, and it conventionally carries units of siemens, formerly called mhos).
