Substance in which all electric charges are bound.
The application of an electric field will not cause a current to flow (as it would in a conductor) but instead a slight movement of the charges polarizes the dielectric.
Dielectrics which are permanently polarized are known as polar dielectrics. Those which lose polarization when there is no applied electric field are, predictibly, known as non-polar dielectrics.
Dielectrics are used in capacitors.