(n.) the forked mark left on something by a lightning strike
The splintering, vein-like pattern often left on timber, earth, or even human skin by a lightning strike is variously known as a keraunograph, keranograph, or ceranograph. Producing these patterns artificially, meanwhile, by deliberately passing electrical currents through wood, is an art form known as keraunography.
At the root of both these words is ceraunos, a Greek word for a thunderbolt. It’s not a particularly well-used root in English today, but it does crop up in a handful of related (and suitably peculiar) words like ceraunics, the branch of physics that deals with the passage of electricity like this; ceraunite, another word for a meteorite, that literally means ‘thunder-stone’; and ceraunoscope, the name of a device for replicating the sound of thunder in a theatre.
The term keraunography appears to have been coined by nineteenth-century folklorists, as a means of collectively discussing the properties which lightning strikes and their patterns were supposed to hold. According to legend, lightning was able to project and sear onto the nearest surfaces the outlines of things unseen to the naked eye—a belief tied in with an event from 1596, when a lightning strike at Wells Cathedral in Somerset apparently burned the outline of a cross onto the bodies of all those nearby.
As the scientists and physicists of the eighteenth and nineteenth centuries began to discover more about electricity, the superstitions surrounding lightning strikes and keraunographs dwindled out of use; indeed one of these early researchers, the German physicist Georg Christoph Lichtenberg, even gave these branching patterns their proper name, Lichtenberg figures.
The word keraunograph itself, however, has survived—and has now, in the twenty-first century, come to be used to the striking process by which such patterns can be created intentionally.