Dielectric is something that is a non conductor of electricity. In scientific or exact terms only 10-6 that is less then a millionth of siemens conductivity or less substances are referred to as dielectric. It can be described as an insulator or non conductor. A dielectric substance can hold electrostatic charges but cannot conduct or allow current to flow through them.
Examples of dielectric materials are glass, plastic and rubber that make for good insulators. In applicability terms or in terms of industrial use dielectrics are classified as good or bad based on certain characteristics. A dielectric that is good should not show variations in its properties when temperatures fail to remain constant or over a broad temperature range. It should consume or draw power itself from the circuit, it should be reasonably stable physically and it should not breakdown when the voltage is high.