The dielectric constant is the ratio of the permittivity of a substance to the permittivity of free space. It is an expression of the extent to which a material concentrates electric flux, and is the electrical equivalent of relative magnetic permeability.(ε): A measure of a substance’s ability to insulate charges from each other. Taken as a measure of solvent polarity, higher ε means higher polarity, and greater ability to stabilize charges. Not the same as dipole moment.
As the dielectric constant increases, the electric flux density increases, if all other factors remain unchanged. This enables objects of a given size, such as sets of metal plates, to hold their electric charge for long periods of time, and/or to hold large quantities of charge. Materials with high dielectric constants are useful in the manufacture of high-value capacitors.
it is important to note that there is no standard symbol for the dielectric constant – you may see it referred to as κ, ε, ε′ or εr. In this TLP κ shall be used to avoid confusion with the absolute permittivity, which may also be given the symbol ε.
The term "dielectric" was coined by William Whewell (from "dia-electric") in response to a request from Michael Faraday. A perfect dielectric is a material with zero electrical conductivity. (cf. perfect conductor), thus exhibiting only a displacement current; therefore it stores and returns electrical energy as if it were an ideal capacitor.