Corona in a transformer is a form of partial discharge in gas-filled voids or at sharp electric field gradients, typically in oil, paper interfaces, or at conductor edges. It occurs when the local electric field exceeds the ionization threshold of the surrounding medium, creating micro-discharges without a full breakdown. Over time, corona produces hydrogen, ozone, and acidic byproducts, degrades paper surfaces, and accelerates insulation aging. It is often an early-stage defect that does not trip protection immediately but shows up clearly in hydrogen DGA trends and acoustic/PD measurements.