Transistor
Definition
A tiny electronic switch and amplifier — the building block of every computer chip, with billions on a single processor.
Technical definition
A three-terminal semiconductor device (e.g., bipolar: base/collector/emitter; MOSFET: gate/source/drain) where a small input signal controls a larger current. As a switch it enables digital logic; as an amplifier it enables analog electronics.
Example
A modern CPU contains tens of billions of transistors, each switching billions of times per second — every calculation you have ever done on a computer ran through them.
Common confusion
A transistor does not amplify power from nothing — it controls a larger current from a power supply using a small input signal. Like a faucet: a small twist of the handle controls a large water flow.