An insulator has poor electrical conduction but an insulator with high polarizability is called a dielectric. Such material can be used not to just reduce the flow of charge, but to store electrical charge. A dielectric material (dielectric for short) is an electrical insulator that can be polarized by an applied electric field.

When a dielectric is placed in an electric field the charged particles within the dielectric slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, positive charges are displaced in the direction of the field and negative charges shift in the opposite direction. This creates an internal electric field in opposition to the applied one and that reduces the overall field within the dielectric itself.

If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient themsleves so that their symmetry axes align with the field lines.

The relative permittivity or dielectric constant (a term deprecated in physics and engineering, but one which is still commonly used in chemistry)

The relative permittivity of a material indicates the energy storing capacity of the material (by means of polarization).

A common example of a dielectric is the electrically insulating material between the metallic plates of a capacitor.

The polarization of the dielectric by the applied electric field increases the capacitor's surface charge for the given electric field strength.

The relative permittivity of a material is its dielectric permittivity expressed as a ratio relative to the permittivity of vacuum.

Relative permittivity is typically denoted as ε_r and is defined as

ε_r = ε/ε₀

where

ε is the complex frequency-dependent absolute permittivity of the material, and

ε₀ is the permittivity of a vacuum

Permittivity is a material property that affects the Coulomb force between two point charges in the material.

Relative permittivity is the factor by which the electric field between the charges is decreased by the material relative to how much it would be decreased by a vacuum. Or you could think of it as being the ratio of the capacitance of a capacitor using that material as a dielectric, compared to a similar capacitor that has vacuum as its dielectric.

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In the equation for a parallel plate capacitor

we could write C = Aε/d

Capacitors - an introduction The parallel plate capacitor Storing Energy in a Capacitor Capacitor Discharge An Experiment to Determine the Capacitance of a Capacitor

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