Bright flashes of light near the horizon without thunder (especially on hot evenings); usually attributed to distant lightning that is reflected by clouds WordNet 3.0 & Open English WordNet

Heat lightning, also known as silent lightning, summer lightning, or dry lightning, is a misnomer used for the faint flashes of lightning on the horizon or other clouds from distant thunderstorms that do not appear to have accompanying sounds of thunder. Wikipedia

Lightning not accompanied by thunder sound Wikidata

The faint lightning flashes of distant thunderstorms on the horizon, without audible thunder. OmegaWiki

More or less vivid and extensive flashes of electric light, without thunder, seen near the horizon, especially at the close of a hot day. Wiktionary

Heat lightning. Wiktionary (translation)