The term "heat lightning" is commonly used to describe lightning from a distant thunderstorm just too far away to see the actual cloud-to-ground flash or to hear the accompanying thunder.
While many people incorrectly think that heat lightning is a specific type of lightning, it is simply the light produced by a distant thunderstorm.
Often, mountains, hills, trees or just the curvature of the earth prevent the observer from seeing the actual lightning flash. Instead, the faint flash seen by the observer is light being reflected off higher-level clouds. Also, the sound of thunder can only be heard for about 10 miles from a flash.