Definition

1. Noun:
   • An illusion that misleads: A deceptive or misleading belief, hope, or goal; a delusive ideal or aspiration.
   • A pale light sometimes seen at night over marshy ground: A phosphorescent light seen hovering or flitting over marshy ground, often attributed to the combustion of gases from decomposed organic matter; also called will-o'-the-wisp or friar's lantern.

Examples of Usage

• As a misleading illusion:
  • The promise of quick riches was nothing but an ignis fatuus, leading many to financial ruin.
  • He pursued the ignis fatuus of perfect happiness, never realizing it was an unattainable dream.
• As a phosphorescent light:
  • Travelers in the swamp were frightened by the eerie glow of an ignis fatuus.
  • The ancient legend spoke of an ignis fatuus that would lead the unwary deep into the treacherous bog.

Advanced Usage

• Literary and Figurative Use: The term is often used in literary and formal contexts to describe something alluring but ultimately deceptive or unattainable.
  • The political candidate's grand promises proved to be an ignis fatuus for the disillusioned voters.

Variants and Related Words

• Will-o'-the-wisp (n): The most common synonym, especially for the light phenomenon. It carries the same figurative meaning of something elusive or deceptive.
• Friar's lantern (n): A less common, archaic synonym for the marsh light.
• Phosphorescent light (n): A general term for light emitted without significant heat, like the marsh gas phenomenon.

Synonyms

• Mirage: An optical illusion or a hope or wish that is impossible to achieve.
• Chimera: A thing that is hoped or wished for but is illusory or impossible to achieve.
• Delusion: A false belief or impression.
• Illusion: A false idea or belief; a deceptive appearance or impression.

Related Idioms and Phrases

• A will-o'-the-wisp: Used identically to "an ignis fatuus" to describe an elusive or deceptive goal.
  • Chasing fame can be like chasing a will-o'-the-wisp.
• A fool's errand: A task that has no hope of success. This relates to the figurative sense of pursuing a deceptive goal.
  • Trying to change his mind was a fool's errand.

Noun

1. an illusion that misleads
2. a pale light sometimes seen at night over marshy ground