VDR(Voltage Dependent Resistor/Varistor).
The VDR is a resistor whose resistance falls as the voltage over it rises towards a spesific point.

Most modern VDRs are made from granulated zinc-oxide that is doped with bismuth, manganese, antimony and other metals and then fused together. The contact surfaces of these grains act as a semiconductor with a voltage drop of about 3V. The total voltage drop of the component depends on the thicknes of the VDR and the size of the grains. At voltages below this, the VDR has a high resistance, but as the voltage pass this the resistance falls rapidly.

Because of this property VDRs can be used to protect against transient high voltages(made by switching off inductive loads or by lightning) by connecting them like this:
    ~  --|_____|-----*------------
  line 1  F1         |            |
                    | | VDR 1   ..|..
                    |_| 150V   :  |  :
                     |         : | | :
   GND --------------*---------: | | : load
                     |         : | | :
                    | | VDR 2  :..|..:
                    |_| 150V      |
  line 2  _____      |            |
    ~  --|_____|-----*------------ 

If the voltage on line 1 gets higher than 150V, VDR1s resistance falls. This shorts line 1 to ground and blows the fuse F1 thereby protecting the load(your computer, TV, stereo or some other important device).

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