Lexicon Impedance

Also called impedance. It is the ratio between Voltage across a component and the Current through the component, expressed in the unit Ohm. The impedance of a component or an entire circuit is usually frequency-dependent, which is why the specification of the Frequency is usually additionally required. At frequency 0 Hz (direct current), impedance and (Active) Resistance are identical. At AC (frequency > 0 Hz), the effects of reactive (energy-storing) components such as capacitors and coils are added.

Because it is always confused:

Resistance (also called effective resistance) is a DC effect.

The impedance (also called impedance) is an alternating current effect

Resistance is thus a special case of impedance at frequency 0 Hz.

Example: a commercially available speaker of nominal impedance 8 ohms consists internally of several components, such as chassis and (in the crossover) coils, capacitors and resistors. These result in an effective impedance, which can be measured with an ohmmeter in the case of direct current. The indication of the nominal impedance does not say that this resistance is 8 Ohm. It means that the impedance over the whole audio frequency range may not fall more than 20% below the nominal impedance. However, it may be as much higher as desired. To check this, measure the impedance at all frequencies from 0Hz to over 20kHz, and plot a graph of impedance versus frequency. The curve must not fall below 8 Ohm - 20% at any point.

It follows: You can't measure impedance with an ohmmeter, because that measures with direct current. An impedance meter measures with alternating current of selectable frequency, and is therefore a more complicated and more expensive device.

See also: Impedance in Wikipedia