ignis fatuus

/ˌˈɪgnəs ˌfætʃəwəs/

A glowing, mysterious light that you see at night in a swamp or marsh is an ignis fatuus. An ignis fatuus is caused by a chemical reaction, but it looks ghostly.

The combination of methane gas and decaying plants that's found in bogs and swamps can result in a glow or pale fire that's visible at dusk or in the dark. One of many names for this is ignis fatuus — it's also known as a will o' the wisp or a friar's lantern. There are numerous myths explaining the ignis fatuus, holding ghosts, fairies, and devils responsible for the spooky lights. In modern Latin, ignis fatuus literally means "foolish fire."

Definitions of ignis fatuus

1. noun

   a pale light sometimes seen at night over marshy ground
   synonyms: friar's lantern, jack-o'-lantern, will-o'-the-wisp

   type of:

   light, visible light, visible radiation

   (physics) electromagnetic radiation that can produce a visual sensation
2. noun

   an illusion that misleads
   synonyms: will-o'-the-wisp

   type of:

   fancy, fantasy, illusion, phantasy

   something many people believe that is false