Semiconductor
Definition
A material — like silicon — whose ability to conduct electricity sits between a conductor and an insulator, and can be precisely controlled. Every computer chip is built on semiconductors.
Technical definition
A crystalline material with a band gap small enough that conductivity can be tuned by doping, temperature, light, or electric fields. Controlled charge-carrier populations enable diodes, transistors, and integrated circuits — the basis of all modern electronics.
Example
Pure silicon barely conducts, but adding one phosphorus atom per million silicon atoms donates free electrons and makes it conduct well. That controllable switch — conducting or not, on demand — is what a transistor does billions of times per second in your phone.
A semiconductor is not a 'bad conductor.' Its value is exactly that its conductivity is adjustable — a copper wire cannot be switched, but a semiconductor can. The magic is control, not the middle-of-the-road conductivity itself.