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 dissipation¹ 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.
External links
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.