A dielectric, or electrical insulator, is a substance that is highly resistant to flow of electric current.
Applications
The use of a dielectric in a capacitor presents several advantages. The simplest of these is that the conducting plates can be placed very close to one another without risk of contact. Also, if subjected to a very high electric field, any substance will ionize and become a conductor. Dielectrics are more resistant to ionization than air, so a capacitor containing a dielectric can be subjected to a higher voltage. Layers of dielectric are commonly incorporated in manufactured capacitors to improve their performance above that of capacitors with only air or a vacuum between their plates, and the term dielectric refers to this application as well as the insulation used in power and RF cables.
Source: CDX Global & Wikipedia - en.wikipedia.org
