Increasing the overall speed to run at a higher clock speed than the manufacturer's specification with an accompanying increase in temperature. It is commonly done by PC enthusiasts who are capable of keeping the processor temperature down through effective cooling techniques such as fans, heatsyncs, and other methods.
Overclocking is often advised against by manufacturers who usually test the processor thoroughly at specific clock speeds. Should the cooling mechanism fail, there is a possibility the silicon in the microchip will begin to liquify, destroying the processor. While newer processors may lower their own clock speed in response to a high-temperature, you should check your documentation to make sure you do not damage the system you seek to improve.
What is safe?
Many system engineers, both in computers and elsewhere, seek to create systems that have as few moving parts as possible and thus increase the Mean Time Between Failure. As such, they may in fact underclock a system so as to remove the fans.
Fans are noisy and using other options to keep processors cool such as very large heatsinks such as in the case of the Intel Xeon processor. See Slashdot article: How to Build a Fast Air-Cooled Quiet PC
In the case of laptops, where low power consumption can increase battery life, processor companies such as Transmeta and Intel's Speed Step technology seek to reduce the power consumption of processors, often on-the-fly.
- Overclocking database
- ArsTechnica's Tweak and Howtos - Overclocking Myths: of yields and electromigration, A brief history of clock: Overclocking mechanics, Celeron Overclocking FAQ
- Open Directory Project - Fans and Cooling Devices