A material that varies in its level of conductivity, not truly an insulator nor truly a conductor. This is done by using the correct material, adding impurities to it, and then using applied voltages. This means that, among several things, they can be used like switches, conducting or not conducting as needed.
A material that is neither a good conductor nor a good insulator.
Grob’s Basic Electronics, 11th Edition by Mitchel E. Schultz
A material that lies between conductors and insulators in its conductive properties.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd