Dielectric

In electromagnetism, a dielectric (or dielectric material or dielectric medium) is an electrical insulator that can be polarized when the insulator is in an electric field.
Dielectric materials include types of glass, mica, bakelite, plexiglass, and kinds of paper. Dielectric materials reduce the overall electric field, like when one of these materials is inserted in a capacitor. This happens because of polarization in dielectric material. Charges are induced in dielectric materials being used which creates an opposite electric field, resulting in the lowering of the ELECTRIC FIELD it is in. Generally, positive charges are induced in negative plates and vice versa, and so a dielectric material reduces the overall electric field. How it does that is normally represented as the "dielectric constant" (generally represented as "k", in equations), as a result POTENTIAL DIFFERENCE is also reduced by "dielectric constant" times, specifically; if dielectric is inserted in capacitors then new capacitance becomes "dielectric constant" times the original capacitance.
Examples (or practical dielectrics).
Dielectric materials can be solids, liquids, or gases. (A high vacuum can also be a useful, nearly lossless dielectric; However its relative dielectric constant is only unity.
Solid dielectrics are perhaps the most commonly used dielectrics in electrical engineering. Many solids are very good insulators. Some examples include porcelain, glass, and most plastics. Air, nitrogen and sulfur hexafluoride are the three most commonly used gaseous dielectrics (or gases that are in use as dielectrics).