An electrical insulator is a material that lacks movable electric charges, and therefore lacks a low-resistance path for the flow of electric current. In electrical insulators, or nonconductors, there is a wide separation between completely filled and completely empty electronic energy levels. Even the outermost electrons are so tightly bound that there is essentially zero electron flow through them when ordinary voltages are applied.
Most non-metallic solids are electrical insulators and some materials, such as mica, glass, and quartz are particularly good insulators as characterized by their high resistivities. Most pure solid ionic substances are good insulators, although impurities or imperfections may introduce semiconductivity.
Compare with electrical conductor.