The Collaborative International Dictionary of English v.0.48:

Transistor \Trans*ist"or\, n. [transfer + resistor, from its
   ability to tranfer a current across a resistor.]
   (Electronics)
   a component used in electronic devices consisting of three
   regions of at least two types of a semiconducting material,
   such as doped silicon, connected to each other and to three
   electrodes in a conducting path so as to modify the current
   or voltage in an electronic circuit.

   Note: Transistors are used in almost all modern electronic
         devices, having replaced the vacuum tube in most
         applications. Since they are based on the electronic
         characteristics of solids, they are called solid-state
         devices. Typically a transistor is composed of p, n,
         and p-type semiconductors in series, or of n, p, and n,
         with the center region being a thin layer between the
         two outer regions. An electronic signal input to the
         central layer may be substantially amplified by such a
         device. In integrated circuits, many thousands of
         transistors may be etched into a single small wafer of
         silicon.
         [PJC]

WordNet (r) 3.0 (2006):

transistor
    n 1: a semiconductor device capable of amplification [syn:
         transistor, junction transistor, electronic
         transistor]

The Free On-line Dictionary of Computing (30 December 2018):

transistor

    A three terminal semiconductor amplifying
   device, the fundamental component of most active electronic
   circuits, including digital electronics.  The transistor was
   invented on 1947-12-23 at Bell Labs.

   There are two kinds, the bipolar transistor (also called the
   junction transistor), and the field effect transistor (FET).

   Transistors and other components are interconnected to make
   complex integrated circuits such as logic gates,
   microprocessors and memory.

   (1995-10-05)