Short for Complementary Metal Oxide Semiconductor Complementary. CMOS is a widely used type of semiconductor. CMOS semiconductors use both NMOS (negative polarity) and PMOS (positive pole) circuits. Because only one type of circuit is on at any given time, CMOS chips require less power than chips using just one type of transistor. This makes them very attractive for use in battery-powered devices, such as portable computers. Personal computers also contain a small amount of battery-powered CMOS memory to hold the date, time, and system setup parameters.