naked flame; open flame
Refers to an uncovered flame, such as a candle, match, or gas burner, often in safety contexts.
実験室では裸火の使用を禁止しています。
The use of naked flames is prohibited in the laboratory.
ろうそくの裸火で部屋がほのかに照らされた。
The room was faintly lit by the naked flame of a candle.
Compound of 裸 (はだか, 'naked') and 火 (ひ, 'fire'), with rendaku voicing of ひ to び. The term straightforwardly describes an uncovered flame.