insulator

Any poor conductor of heat, sound, or electricity. Most substances lacking free (mobile) electrons, such as non-metals, are electrical or thermal insulators that resist the flow of electricity or heat through them. Plastics and rubber are good insulators. Usually, devices of glass or porcelain, called insulators, are used for insulating and supporting overhead wires.

The electrons in the outer shells of the atoms of insulators are strongly attracted by the positively-charged nucleus. When external energy is supplied, these electrons are not easily removed from the outer shells of the atoms, which gives these materials their insulating properties.