Sheet lightning
From Glossary of Meteorology
sheet lightning
(Also called luminous cloud.) 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.
Thus, sheet lightning is no unique form of lightning but only one manifestation of ordinary lightning types in the presence of obscuring clouds.
Compare heat lightning.
Compare heat lightning.