CMOS (Pronounced Sea-Moss), stands for Complementary Metal Oxide Semiconductor.

CMOS is a widely used type of semiconductor, that uses both NMOS (negative polarity) and PMOS (positive polarity) circuits. Since only one of the circuit types is on at any given time, CMOS chips require less power than chips using just one type of transistor. This makes them particularly attractive for use in battery-powered devices, such as portable computers, like Laptops and PDA's.

Personal Computers also contain a small amount of battery-powered CMOS memory to hold the Date, Time, System Setup Parameters, and Disk Information, as well as CPU, RAM, and FSB Frequency settings.