CMOS (pronounced "see-moss"), which stands for complementary metal-oxide-semiconductor, is a major class of integrated circuits. CMOS chips include microprocessor, microcontroller, static RAM, and other digital logic circuits. The central characteristic of the technology is that it only uses significant power when its transistors are switching between on and off states. Consequently, CMOS devices use little power and do not produce as much heat as other forms of logic. CMOS also allows a high density of logic functions on a chip.

The word "complementary" refers to the fact that the design uses pairs of transistors for logic functions, only one of which is switched on at any time.

The phrase "metal-oxide-semiconductor" is a reference to the nature of the fabrication process originally used to build CMOS chips. That process created field effect transistors having a metal gate electrode placed on top of an oxide insulator, which in turn is on top of a semiconductor material. Instead of metal, today the gate electrodes are almost always made from a different material, polysilicon, but the name CMOS nevertheless continues to be used for the modern descendants of the original process.