1. A discharge of atmospheric electricity, accompanied by a vivid flash of light, commonly from one cloud to another, sometimes from a cloud to the earth. The sound produced by the electricity in passing rapidly through the atmosphere constitutes thunder.
2. The act of making bright, or the state of being made bright; enlightenment; brightening, as of the mental powers. Ball lightning, a rare form of lightning sometimes seen as a globe of fire moving from the clouds to the earth. Chain lightning, lightning in angular, zigzag, or forked flashes. Heat lightning, more or less vivid and extensive flashes of electric light, without thunder, seen near the horizon, especially. at the close of a hot day. Lightning arrester, a luminous beetle. See firefly. Lightning conductor, a lightning rod. Lightning glance, a quick, penetrating glance of a brilliant eye. Lightning rod, a metallic rod set up on a building, or on the mast of a vessel, and connected with the earth or water below, for the purpose of protecting the building or vessel from lightning. Sheet lightning, a diffused glow of electric light flashing out from the clouds, and illumining their outlines. The appearance is sometimes due to the reflection of light from distant flashes of lightning by the nearer clouds.