Underclocking is the practice of changing a microprocessor's speed settings to run at a lower clock speed than the manufacturer's specification. The idea is generally to decrease heat production¹ and/or decrease electricity consumption, but this invariably comes at the expensive 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 alow a system to run with smaller (and therefore quieter) cooling fan or without one at all.