From AMS Glossary
(Sometimes called breakdown potential.) A measure of the resistance of a dielectric to electrical breakdown under the influence of strong electric fields; usually expressed in volts per meter.
The dielectric strength of dry air at sea level pressures is about 3 000 000 V m-1. The exact value for air depends upon geometry of the electrodes between which the electric field is established, upon the humidity, and upon whether or not water drops are present in the air.