Underclocking

Underclocking is the practice of modifying a microprocessor's speed settings to run at a lower clock speed than the manufacturer's specification. The purpose is generally to decrease heat dissipation[?]¹ and/or decrease electricity consumption, but this invariably comes at the expense of system performance.

While underclocking is a relatively rare practice, overclocking is not. Basically, underclocking is the reverse of overclocking.

 ¹ This can be done for the purposes of increasing system stability in high-heat environments, or to allow a system to run with a lower airflow (and therefore quieter) cooling fan or without one at all.