Partial Discharge is "A localized electrical discharge that only partially bridges the insulation between conductors and which can or cannot occur adjacent to a conductor" ---- quoted from IEC60270.
When the Voltage stress exceeds the breakdown strength of that portion of the insulating material, a Partial Discharge begins and continues to deteriorate that insulation.
When Partial discharges occur, various physical and chemical changes may happen, which produce emissions that we can detect, measure, diagnose and localize to prevent insulation failure of Medium and High Voltage electrical equipment.
PD occurs in Solid, Liquid or Gaseous insulating mediums. PD can also occur in the form of Corona, Surface Tracking, or floating electrodes (metal to metal discharge) causing degradation of the insulation. Once PD begins it will always get worse. 80% of electrical fires are caused by insulation breakdown or failure and PD is the very first indication that insulation is breaking down. NFPA 70B states that insulation breakdown is the number one cause of electrical failures.