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sheet lightning[′shēt ¦līt·niŋ]
A diffuse, but sometimes fairly bright, illumination of those parts of a thundercloud that surround the path of a lightning flash, particularly a cloud discharge or cloud-to-cloud discharge. Also known as luminous cloud.
Slang for a lightning discharge occurring within the anvil of a thunderstorm. It is characterized by one or more channels that appear to crawl along the underside of the anvil. They typically appear during the weakening or dissipating stage of the parent thunderstorm or during an active mesoscale convective system. Also called sheet lightning.