The dielectric constant does not have units or dimensions because it expresses the ratio of the permittivity of a substance to that of free space or a vacuum. The dielectric constant is a property of electrical insulators, and the symbol for the dielectric constant is the Greek letter kappa. Another name for the dielectric constant is "relative permittivity."
Permittivity describes the distortion of the atomic charge in an insulator in the presence of an electric field. A vacuum, by definition, has a permittivity value of one, and it is this value that is used as the denominator in the ratio. The dielectric constant is used to characterize capacitors, which are capable of storing electric charge. Every material has a dielectric constant, and it is the material that makes up the dielectric that determines how effective the capacitor is at storing charge.
Dry air, a perfect vacuum and pure, dry gases have low dielectric constants. Materials with moderate dielectrics include glycerin, glass and water. Metal oxides, such as aluminum oxide and titanium dioxide, have high dielectric constants. Whether a person wants a low, moderate or high dielectric material depends on the application, because each serves a different purpose. The dielectric constant is always greater than or equal to one because the numerator is always greater than or (in the case of air) equal to the denominator. The higher the dielectric constant, the more charge that material is capable of storing.