Semiconductor
Beginner
A material, like silicon, that carries electricity better than glass but much worse than metal. We can change how much it carries on purpose.
Novice
A solid with a full valence band and an empty conduction band separated by a modest band gap (1.12 eV in silicon). Heat, light and added impurities can put carriers into the bands, so its conductivity can be controlled over many orders of magnitude.
Expert
Distinguished from an insulator by degree: small enough that doping sets carrier densities at room temperature. Silicon wins on its native oxide, cost and crystal quality, not on mobility or band gap.
Explained in Bands, doping and the p-n junction (Transistors).