An electrical insulator, like air, glass, or other non-conducting mediums and materials.

Disruptive discharge

A disruptive discharge is what Nikola Tesla called the event of a capacitor, charged to its maximum capacity, discharging all of its stored energy with extreme suddenness through a spark gap, creating oscillating currents in the circuit. This idea was initially developed by the likes of Joseph Henry and Lord Kelvin and was later put to great practical use by Tesla in his high voltage, high frequency devices.