semiconductor
A semiconductor is a material that can't quite decide whether to conduct electricity or block it — and that indecision is exactly what makes it useful. A metal like copper always conducts; glass never does. Silicon sits in between, like a doorway whose width you can dial: nudge it one way and current pours through, nudge it back and the door slams shut. You don't get a fixed material so much as a knob.
The trick is doping: stir in a few atoms of phosphorus per million and you flood silicon with spare electrons (n-type); use boron instead and you create electron-hungry "holes" (p-type). Both shifts are minute but precisely controlled. Press an n-type and a p-type region together and you get a junction that passes current one way and refuses it the other — a diode. Stack such junctions with a third terminal to steer them, and you get a transistor: a switch with no moving parts, the building block from which every chip is made.
"Semiconductor" gets used loosely to mean the whole chip industry, but strictly it's the material class — defined by a band gap small enough to cross with a little heat, light, or voltage (silicon's is about 1.1 eV). A common mix-up: the point isn't that doped silicon conducts better in some fixed way; it's that you can switch its conductivity on and off, which is exactly what lets a transistor compute.