A transistor is a semiconductor device that is commonly used to amplify or switch electronic signals. It consists of three terminals, normally referred to as the emitter, base and collector. The transistor operates in four main states: cutoff, forward active, reverse active and saturation. It acts as an electrical switch when forward biased, meaning a current is allowed to flow between the two terminals. When it is reverse biased, meaning the current flows in the opposite direction, the device is turned off. Transistors are used in amplifying circuits to amplify voltage, current, or even power. They are also used in digital logic circuits to switch between two or more stable states.