Transistor
Beginner
A tiny electric switch with no moving parts. Electricity on its control connection turns it on or off.
Novice
A semiconductor device in which a small signal on one terminal controls the current between two others. Digital chips use it as an on/off switch; analog circuits use it as an amplifier.
Expert
In digital CMOS, almost always a MOSFET used as a voltage-controlled switch with finite on-resistance, a gate capacitance to charge, and nonzero off-current. Bipolar transistors survive in analog, RF and some I/O circuits.