A dielectric is a nonconductor of electric charge in which an applied electric field causes a displacement of charge but not a flow of charge. Electrons within the atoms of a dielectric are, on average, displaced by an applied field with respect to the nucleus, giving rise to a dipole that has an electric moment in the direction of the field. The resulting stress within the dielectric is known as the electric polarization (P) and is defined by P = D - Eε0, where D is the displacement, E is the electric field strength, and ε0 is the electric constant.
The dielectric constant is known called the relative permittivity. The dielectric strength is the maximum potential gradient that can be applied to a material without causing it to break down. It is usually expressed in volts per millimeter.