Plain English Meaning
A sudden, powerful spark of electricity in the sky, caused when built-up electrical charges jump across the air to balance out. It can jump from cloud to cloud or from a cloud down to the ground.
Full Definition
A high-energy electrical discharge that occurs when the difference in electrical charge between two regions becomes large enough to overcome the insulating resistance of the air between them. Lightning may discharge between two clouds, between different parts of the same cloud, or between a cloud and the ground.
Context Anchor
Pilots encounter lightning in weather briefings, in-flight weather reports, and visually when deciding whether to avoid or delay flight near storms.
Derivation
From Old English 'leoht' (light), referring to the bright flash produced by the discharge. The visible flash is the result, not the cause -- the actual event is the electrical discharge itself.
Grounding Statement
If you can see lightning near your route, active storm energy is nearby, even if the rain has not reached the airport yet.
Intuition Check
Do not treat lightning as only a bright flash in the distance. For pilots, lightning means thunderstorm activity may be close enough to affect the flight.
Example Sentence 1
The pilot diverted around the cell after seeing repeated lightning flashes in the towering cumulus ahead.
Example Sentence 2
Aircraft are designed to withstand a lightning strike without catastrophic failure.