Intermittent flashes of light across the sky on warm summer evenings, not accompanied by thunder. It is thought to be cloud reflections of distant lightning.
Ordinary lightning that is too far away for its thunder to be heard; a misnomer.
Heat lightning refers to lightning that illuminates the sky, but is too far away for its thunder to be heard. It is not a true form of lighting, but rather a way to describe the sheet-like flash that is observed.
Lightning that can be seen, but is too far away to be heard.
Non-technically, the luminosity observed from ordinary lightning too far away for its thunder to be heard. There has arisen a popular misconception that the presence of diffuse flashes in the apparent absence of thunderclouds implies that lightning is somehow occurring in the atmosphere merely as a result of excessive heat.
Lightning that appears as a glowing flash on the horizon. It is actually lightning occurring in distant thunderstorms, just over the horizon and too far away for thunder to be heard.