Final answer:
An insulator in an electric field is called a dielectric, which is a material that does not conduct electricity but can become polarized in the presence of an electric field.
Step-by-step explanation:
An insulator in an electric field is called a dielectric. A dielectric is an electrically insulating material that becomes polarized when placed in an electric field. This polarization occurs because the material's electrons are displaced slightly, creating tiny induced dipoles within the material, although the electrons do not flow freely as they would in a conductor.
In relation to the ionosphere, while this layer of Earth's atmosphere does contain charged particles and has an electric field associated with it, it is not an example of a dielectric material since it is not used to insulate or separate electric charges in the context of electric components or circuits. Instead, good dielectrics are materials like glass, porcelain, or certain polymers which can effectively prevent the flow of electrical charge while becoming polarized in the presence of an electric field.