What is meant by MOS transistor?

The transistor consists of a stack of the conducting gate, an insulating layer of silicon dioxide (SiO2) and the silicon wafer (also called the substrate, body, or bulk). Gate of transistor is built from metal, so the stack is called metal oxide- semiconductor (MOS).

Transistor operation is controlled by electric field so the device is also called Metal Oxide Semiconductor Field Effect Transistor (MOSFET).