Overclocking is the technique used to increase the speed of a device beyond stock. ‘Stock' is the name given to the default specifications of a device. Once you have altered the device it is no longer considered stock unless it is returned to its initial specifications. Overclocking describes the alteration of clock speed above stock (hence the name overclocking) of any given device, usually RAM and CPUs, but also other components such as GPUs and PCI busses. When a devices clock speed is decreased from stock (or from an overclock), it is called underclocking.