A material that resists the flow of electricity. This is because the electrons in the materials are tightly bound to the nucleus, meaning it will take a lot of energy to pull that electron away. This gives insulators an extremely large band gap between the valence band and the conduction band.
A material that does not allow current to flow when voltage is applied, because of its high resistance.
Grob’s Basic Electronics, 11th Edition by Mitchel E. Schultz
A material that does not conduct current.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd