Complementary Metal Oxide Semiconductor: a process that uses both N- and P-channel devices in a complimentary fashion to achieve small geometries and low power consumption.
CMOS is commonly used on PC motherboards and is a battery powered chip that stores the BIOS settings and hardware configuration information as well as the current time and date. The chip itself is either a separate microchip mounted on the motherboard or build into the south bridge.
A CR 2032 3V coin shaped battery is often mounted on a computer’s motherboard to power the CMOS RAM.