Transistor

The transistor is a solid state semiconductor device which can be used for amplification, switching, voltage stabilization, signal modulation and many other functions. It acts as a variable valve which, based on its input current (BJT) or input voltage (FET), allows a precise amount of current to flow through it from the circuit's voltage supply.
In essence, a transistor have three terminals. A current or voltage applied through/across two terminals controls a larger current through the other terminal and the common terminal. In analog circuits, transistors are used in amplifiers. Analog circuits comprise audio amplifiers, stabilised power supplies and radio frequency amplifiers. In digital circuits, transistors function basically as electrical switches. Digital circuits include logic gates, RAM (random access memory) and microprocessors.
Transistor was also the common name in the sixties for a transistor radio, a portable radio that used transistors (rather than vacuum tubes) as its active electronic components. This is still one of the dictionary definitions of transistor.