Varistors 

Electrical resistances that behave voltage-dependent are called varistors (variable resistors). Variable because above a certain voltage (threshold voltage) the differential resistance suddenly decreases.  Varistors are also abbreviated VDR (voltage dependent resistor) or MOV (metal oxide varistor).  The components are characterized by a high durability against repeated high pulse currents.

In order for a varistor to optimally fulfil its primary function as overvoltage protection (high energy absorption capacity, fast response to voltage peaks), it must be optimally designed for its area of application (type and extent of overvoltage situations, acceptable forward voltage, safety standards, etc.).