What does fallacy mean?
A fallacy is a false or mistaken belief, especially one that many people accept as true. The word also has a precise technical meaning in logic, where it denotes a flaw in reasoning — a defect in the structure of an argument that makes its conclusion unreliable even if it sounds persuasive. Common fallacies have been named and studied since antiquity: appealing to emotion instead of evidence, attacking an opponent's character rather than their argument (the ad hominem fallacy), or assuming that what is true of parts must be true of the whole. Calling something a fallacy implies not merely that it is wrong, but that it is wrong in a way that can be identified and exposed through careful reasoning. The word derives from Latin fallacia, meaning deceit or trickery, from fallere, to deceive. It is chiefly used in formal and intellectual contexts — philosophy, rhetoric, law, and everyday argumentation — and remains one of the most useful terms for anyone who wants to reason clearly.
nounA false or mistaken belief, especially one widely held; in logic, a flaw in reasoning that renders an argument invalid or unsound.
- A false or mistaken idea or belief, particularly one held by many people.
- In logic: defective reasoning, or an argument whose conclusion does not follow from its premises.
"It is a common fallacy that goldfish have a three-second memory."
"The belief that cracking your knuckles causes arthritis is a persistent fallacy."
"Her essay dismantled the argument by exposing the ad hominem fallacy at its heart."
Standard English plural; the y changes to -ies.
"The textbook devotes a chapter to the most common fallacies in political rhetoric."
Aristotle catalogued over a dozen logical fallacies more than 2,000 years ago — and we still fall for every single one of them today.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of fallacy
Fallacy comes from Latin fallacia, meaning "deception" or "trick," derived from fallere, "to deceive." It entered English via Old French in the late Middle English period, originally carrying strong connotations of deliberate deceit before narrowing toward mistaken belief and faulty reasoning. Its Latin root also underlies related words such as "fallacious," "fallible," and "infallible."
Related word forms
How fallacy is actually used
"Fallacy" carries a mildly formal, intellectual register and is most at home in academic writing, debate, and critical discussion. In informal speech, words like "myth" or "mistaken belief" are more common. The plural is "fallacies."
Easily confused with fallacy
A fatality is a death resulting from an accident or disaster, whereas a fallacy is a false belief or flawed argument.