Core voltage

From TheBestCaseScenario

One of the very important factors in Overclocking a processor is it's Core Voltage, this has to be increased usually to make sure that processors with higher FSBs can handle this increase in speed.

On most modern processors going higher than 1.6-1.65v is a NO, this can easily damage a CPU, only extreme overclockers should push them so far, even then it's best to try to keep this as low as possible because the more Volts in a component the more heat it will be putting out, so make sure you have decent cooling.

If a processor is unstable at higher clock speeds, increase the voltage slowly until it becomes stable. Usually people increase this before they start the overclocking then when they have the Core clock speed they want they begin to drop the voltage to see if it can run lower and cooler.