Metal Oxide Semiconductor
Metal Oxide Semiconductor (usually given as the acronym, MOS) is a transistor family using field-effect transistors. A subset of MOS, Complementary MOS (usually CMOS), built out of paired NPN and PNP MOS transistors, is now the most common form.
They are now not usually fabricated as discrete devices, but rather form the active elements in integrated circuits. First used in dynamic RAM main memory, they are now ubiquitously used in computers, due to their low power consumption, which reduces cooling issues.
The name is now something of a misnomer, as the basic material has always been silicon, and the insulating layers are no longer oxides, but dielectric materials chosen for specific properties.