http://www.kpsec.freeuk.com/imped.htm

Impedance

Impedance, Z =  V  I

Resistance, R =  V  I

V = voltage in volts (V) I  = current in amps (A) Z = impedance in ohms () R = resistance in ohms ()

Impedance (symbol Z) is a measure of the overall opposition of a circuit to current, in other words: how much the circuit impedes the flow of current. It is like resistance, but it also takes into account the effects of capacitance and inductance. Impedance is measured in ohms, symbol .

Impedance is more complex than resistance because the effects of capacitance and inductance vary with the frequency of the current passing through the circuit and this means impedance varies with frequency! The effect of resistance is constant regardless of frequency.

The term 'impedance' is often used (quite correctly) for simple circuits which have no capacitance or inductance - for example to refer to their 'input impedance' or 'output impedance'. This can seem confusing if you are learning electronics, but for these simple circuits you can assume that it is just another word for resistance.

Four electrical quantities determine the impedance (Z) of a circuit:
resistance (R), capacitance (C), inductance (L) and frequency (f).

Impedance can be split into two parts:

• Resistance R (the part which is constant regardless of frequency)
• Reactance X (the part which varies with frequency due to capacitance and inductance)

For further details please see the section on Reactance below.

The capacitance and inductance cause a phase shift* between the current and voltage which means that the resistance and reactance cannot be simply added up to give impedance. Instead they must be added as vectors with reactance at right angles to resistance as shown in the diagram.

* Phase shift means that the current and voltage are out of step with each other. Think of charging a capacitor. When the voltage across the capacitor is zero, the current is at a maximum; when the capacitor has charged and the voltage is at a maximum, the current is at a minimum. The charging and discharging occur continually with AC and the current reaches its maximum shortly before the voltage reaches its maximum: so we say the current leads the voltage. 

Input Impedance Z_IN

Input impedance (Z_IN) is the impedance 'seen' by anything connected to the input of a circuit or device (such as an amplifer). It is the combined effect of all the resistance, capacitance and inductance connected to the input inside the circuit or device.It is normal to use the term 'input impedance' even for simple cases where there is only resistance and the term 'input resistance' could be used instead. In fact it is usually reasonable to assume that an input impedance is just resistance providing the input signal has a low frequency (less than 1kHz say).
The effects of capacitance and inductance vary with frequency, so if these are present the input impedance will vary with frequency. The effects of capacitance and inductance are generally most significant at high frequencies.
Usually input impedances should be high, at least ten times the output impedance of the circuit (or component) supplying a signal to the input. This ensures that the input will not 'overload' the source of the signal and reduce the strength (voltage) of the signal by a substantial amount.

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Output Impedance Z_OUT

The equivalent circuit of any output

The output of any circuit or device is equivalent to an output impedance (Z_OUT) in series with a perfect voltage source (V_SOURCE). This is called the equivalent circuit and it repesents the combined effect of all the voltage sources, resistance, capacitance and inductance connected to the output inside the circuit or device. Note that V_SOURCE is usually not the same as the supply voltage Vs.It is normal to use the term 'output impedance' even for simple cases where there is only resistance and the term 'output resistance' could be used instead. In fact it is usually reasonable to assume that an output impedance is just resistance providing the output signal has a low frequency (less than 1kHz say).
The effects of capacitance and inductance vary with frequency, so if these are present the output impedance will vary with frequency. The effects of capacitance and inductance are generally most significant at high frequencies.

Usually output impedances should be low, less than a tenth of the load impedance connected to the output. If an output impedance is too high it will be unable to supply a sufficiently strong signal to the load because most of the signal's voltage will be 'lost' inside the circuit driving current through the output impedance Z_OUT. The load could be a single component or the input impedance of another circuit.

The load can be a single component or the input impedance of another circuit

• Low output impedance, Z_OUT << Z_LOAD
  Most of V_SOURCE appears across the load, very little voltage is 'lost' driving the output current through the output impedance. Usually this is the best arrangement.
• Matched impedances, Z_OUT = Z_LOAD
  Half of V_SOURCE appears across the load, the other half is 'lost' driving the output current through the output impedance. This arrangement is useful in some situations (such as an amplifier driving a loudspeaker) because it delivers maximum power to the load. Note that an equal amount of power is wasted driving the output current through Z_OUT, an efficiency of 50%.
• High output impedance, Z_OUT >> Z_LOAD
  Only a small portion of V_SOURCE appears across the load, most is 'lost' driving the output current through the output impedance. This arrangement is unsatisfactory.

http://dictionary.reference.com/browse/impedance

im·ped·ance

  [im-peed-ns]  Show IPA

noun

1.

Electricity . the total opposition to alternating current by anelectric circuit, equal to the square root of the sum of thesquares of the resistance and reactance of the circuit andusually expressed in ohms. Symbol:  Z

2.

Also called mechanical impedance. Physics. the ratio of theforce on a system undergoing simple harmonic motion to thevelocity of the particles in the system.

3.

something that impedes; an obstacle or hindrance.

Origin: 
1886; impede + -ance; term introduced by O. Heaviside

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imped.html

Impedance

While Ohm's Law applies directly to resistors in DC or in AC circuits, the form of the current-voltage relationship in AC circuits in general is modified to the form:

where I and V are the rms or "effective" values. The quantity Z is called impedance. For a pure resistor, Z = R. Because the phase affects the impedance and because the contributions of capacitors and inductors differ in phase from resistive components by 90 degrees, a process like vector addition (phasors) is used to develop expressions for impedance. More general is the complex impedancemethod.