Plain English Meaning
A glowing blue or green light that can appear on the outside of an aircraft when flying through electrically charged air. It looks dramatic but does no damage, though it usually makes the radios crackle with static.
Full Definition
A harmless electrical discharge that appears as a bluish or greenish glow on an aircraft's exterior, typically on sharp edges such as wingtips, propeller tips, antennas, and windshield frames, when the aircraft flies through air carrying a strong electrical charge. It occurs when the electrical potential between the aircraft and the surrounding air becomes high enough to ionize the air, but not high enough to produce a lightning strike. It is often accompanied by loud static on the radios.
Context Anchor
Seen or discussed in instrument flying, thunderstorm avoidance, and reception problems caused by electrical activity around the aircraft.