an insulator

An insulator is a material that almost refuses to carry current — it blocks charge instead of moving it. This is the rubber around a wire, the plastic case of a plug, the ceramic disc on a power-line tower, the glass of a window. If a conductor is an open highway for electrons, an insulator is a wall: the electrons are locked tightly into their bonds and have nowhere free to go.

In band-theory terms, an insulator has a completely FILLED valence band and a completely EMPTY conduction band separated by a LARGE band gap, typically more than about 3 or 4 electron-volts. To make current flow you must lift an electron all the way across that gap into the empty conduction band, and at ordinary temperatures almost none have enough thermal energy to make the jump. So the carrier count n stays essentially zero and the conductivity is minuscule — often below 10^-10 S/m and stretching down past 10^-16 S/m for materials like quartz or good polymers. Their bonding is usually ionic (like table salt or alumina) or strong covalent (like diamond), both of which pin every electron.

Insulators earn their keep two ways: they keep current where it belongs (the sheath on a cable), and they store charge and energy without leaking it (the dielectric inside a capacitor). But no insulator is perfect. Push the voltage high enough and the electric field can rip electrons loose all at once — dielectric breakdown — and the material suddenly conducts, often destroying itself (a spark through air, a punctured capacitor). The field it can survive before this happens is its dielectric strength, a key design limit.