The ability of a dielectric to resist dielectric breakdown.(i.e. the ability of the dielectric to resist electric charge without being destroyed). It is usually measured in volts per unit thickness. Most plastics and mica have exceptionally high dielectric strengths, ranging from 500,000 to 4,000,000 volts/inch. Air has a fair dielectric strength of around 25,000 volts/inch. A perfect vacuum has an infinite dielectric strength (i.e. it will never break down). However, a partial vacuum has a lower dielectric strength than regular air.
Dielectric strength is not to be confused with dielectric constant, which is the factor by which capacitance is increased when using that dielectric.