A dielectric is a nonconducting substance, i.e. an insulator. The term was coined by William Whewell in response to a request from Michael Faraday. Although "dielectric" and "insulator" are generally considered synonymous, the term "dielectric" is more often used to describe materials where the dielectric polarization is important, such as the insulating material between the metallic plates of a capacitor, while "insulator" is more often used when the material is being used to prevent a current flow across it.