(i) Dielectric strength: The voltage which a dielectric material can withstand without breaking down (without losing its dielectric property) is called its dielectric strength. It is represented by kV/mm or kV/cm. e.g. dielectric strength of air is @ 30 kV/cm or 3 kV/mm.
(ii) Breakdown Voltage: The voltage at which the dielectric material breaks down (Start conducting or is no longer an insulator) for a specified thickness, is its breakdown voltage.