The story
So called because it is common on hot summer nights and is sometimes popularly thought to be caused by heat rather storms.
Etymology from Wiktionary, CC BY-SA 4.0.
heat lightning
Today it means: Lightning and/or thunder which the observer attributes to heat rather than a storm (e.g. because the storm/rain is too distant to be seen, or because there is no rain, as with dry lightning). Full definition →