Overclocking is configuration of computer hardware components to operate faster than certified by the original manufacturer, with "faster" specified as clock frequency in megahertz (MHz) or gigahertz (GHz). Commonly operating voltage is also increased to maintain a component's operational stability at accelerated speeds. However semiconductor devices operated at a higher frequencies and voltages generate additional heat, so most overclocking attempts increase power consumption and heat. An overclocked device may be unreliable or fail completely if the additional heat load is not removed or power delivery components cannot meet increased power demands. Many device warranties state that overclocking and/or over-specification voids any warranty.
The purpose of overclocking is to gain additional performance from a given component by increasing its operating speed. Normally, on modern systems, overclocking is targeted at increasing the performance of a major chip or subsystem, such as the main processor or graphics controller, but other components, such as system memory (RAM) or system buses (generally on the motherboard), are commonly involved. The trade-offs are an increase in power consumption (heat) and fan noise (cooling) for the targeted components. Most components are designed with a margin of safety to deal with operating conditions outside of a manufacturer's control; examples are ambient temperature and fluctuations in operating voltage. Overclocking techniques in general aim to "trade" this safety margin by setting the device to run in the "higher end" of the margin, with the understanding that temperature and voltage must be more strictly monitored and controlled by the user as the remaining "safety cushion" is reduced. Examples are that operating temperature would need to be more strictly controlled with increased cooling, as the part will be less tolerant of increased temperatures at the higher speeds; also base operating voltage may be increased to compensate for unexpected voltage drops and to strengthen signalling and timing signals, as low-voltage excursions are more likely to cause malfunctions at higher operating speeds.
While most modern devices are fairly tolerant of overclocking, all devices have finite limits, generally for any given voltage most parts will have a maximum "stable" speed where they still operate correctly. Past this speed the device starts giving incorrect results, which can cause malfunctions and sporadic behavior in any system depending on it. While in a PC context the usual result is a system crash, more subtle errors can go undetected, which over a long enough time can give unpleasant surprises such as data corruption (incorrectly calculated results, or worse writing to storage incorrectly) or the system failing only during certain specific tasks (general usage such as internet browsing and word processing appear fine, but any application wanting advanced graphics crashes the system).
At this point an increase in operating voltage of a part may allow more headroom for further increases in clock speed, but increased voltage can also significantly increase heat output. At some point there will be a limit imposed by the ability to supply the device with sufficient power, the user's ability to cool the part, and the device's own maximum voltage tolerance before it achieves destructive failure. Overzealous use of voltage and/or inadequate cooling can rapidly degrade a device's performance to the point of failure, or in extreme cases outright destroy it.
The speed gained by overclocking depends largely upon the applications and workloads being run on the system, and what components are being overclocked by the user; benchmarks for different purposes are published