A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. The discharge appears in cases where the voltage exceeds a critical value, but an electric arc cannot form. Instead, the discharge appears as a colored glow around an object.

The corona discharge represents a local region where the air (or other fluid) has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona discharge occurs at locations where the strength of the electric field (potential gradient) around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same mechanism as in a gas discharge lamp and in glow discharge, namely, via a combination of bremsstrahlung radiation and changes in electronic state that produce discrete spectral lines. Corona discharges can also happen in thunderstorms or other electrically-active weather, where objects like ship masts or airplane wings have a charge significantly different from the air around them (see St. Elmo's fire).

In many high-voltage applications, corona is an unwanted side effect. Corona discharge from high-voltage electric power transmission lines constitutes an economically significant waste of energy for utilities. In high-voltage equipment like cathode-ray-tube televisions, radio transmitters, X-ray machines, and particle accelerators, the current leakage caused by coronas can constitute an unwanted load on the circuit. In the air, coronas generate gases such as ozone (O₃) and nitric oxide (NO), and in turn, nitrogen dioxide (NO₂), and thus nitric acid (HNO₃) if water vapor is present. These gases are corrosive and can degrade and embrittle nearby materials, and are also toxic to humans and the environment.

Corona discharges can often be suppressed by improved insulation, corona rings, and making high-voltage electrodes in smooth rounded shapes.

Corona discharge can also be useful. Applications for controlled corona discharges include air filtration machines, photocopiers, and ozone generators.

A corona discharge is a process by which a current flows from an electrode with a high potential into a neutral fluid, usually air, by ionizing that fluid so as to create a region of plasma around the electrode. The ions generated eventually pass the charge to nearby areas of lower potential, or recombine to form neutral gas molecules.

When the potential gradient (electric field) is large enough at a point in the fluid, the fluid at that point ionizes and it becomes conductive. If a charged object has a sharp point, the electric field strength around that point will be much higher than elsewhere. Air near the electrode can become ionized (partially conductive), while regions more distant do not. When the air near the point becomes conductive, it has the effect of increasing the apparent size of the conductor. Since the new conductive region is less sharp, the ionization may not extend past this local region. Outside this region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized.

Along with the similar brush discharge, the corona is often called a "single-electrode discharge", as opposed to a "two-electrode discharge"—an electric arc. A corona forms only when the conductor is separated widely enough from conductors at the opposite potential that an arc cannot jump between the two. If the geometry and gradient are such that the ionized region continues to grow until it reaches another conductor at a lower potential, a low resistance conductive path between the two will be formed, resulting in an electric spark or electric arc, depending upon the source of the electric field. If the source continues to supply current, a spark will evolve into a continuous discharge called an arc.