will-o'-the-wisp; phosphorescent light
Refers to a mysterious, flickering light seen at night over marshes or graveyards, traditionally attributed to supernatural causes but scientifically explained as spontaneous combustion of marsh gases. The term is somewhat literary or folkloric.
I heard a story about seeing a will-o'-the-wisp in the graveyard at night.
Will-o'-the-wisps are scientifically considered a spontaneous combustion phenomenon.
From 燐 (phosphorus) + 火 (fire). The term likely arose from the observation that decaying organic matter can produce phosphorescent light, which was historically interpreted as supernatural.