fallacy

American  

[fal-uh-see] / ˈfæl ə si /

noun

plural

fallacies

1. a deceptive, misleading, or false notion, belief, etc..

   That the world is flat was at one time a popular fallacy.

   Synonyms:

   misapprehension, delusion, misconception

2. a misleading or unsound argument.

3. deceptive, misleading, or false nature; erroneousness.

4. Logic. any of various types of erroneous reasoning that render arguments logically unsound.

5. Obsolete. deception.

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fallacy British  

/ ˈfæləsɪ /

noun

1. an incorrect or misleading notion or opinion based on inaccurate facts or invalid reasoning

2. unsound or invalid reasoning

3. the tendency to mislead

4. logic an error in reasoning that renders an argument logically invalid

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fallacy Cultural  

1. A false or mistaken idea based on faulty knowledge or reasoning. For example, kings who have divorced their wives for failing to produce a son have held to the fallacy that a mother determines the sex of a child, when actually the father does. (See sex chromosomes.)

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Etymology

Origin of fallacy

First recorded in 1350–1400; from Latin fallācia “a trick, deceit,” from fallāc- (stem of fallāx ) “deceitful” + -ia -y ³; replacing Middle English fallace, from Middle French

Explanation

A fallacy is a misleading argument or belief based on a falsehood. If you oppose state testing in schools, you think it is a fallacy that educational quality can be measured by standardized tests. Fallacy comes from the Latin fallacia, for deceit. It technically means a flaw in an argument that makes it deceptive or misleading. In poetry, the "pathetic fallacy" is the false idea that things like rocks or stars have human feelings (pathos). Fallacy can also be used more generally for any false statement or idea. Some synonyms are misconception and error.

Example Sentences

The sunk-cost fallacy tempts people to pay escalating premiums because they’ve paid for so long.

From MarketWatch • Nov. 15, 2025

That would be a fallacy, but Mr. Bottum didn’t commit it.

From The Wall Street Journal • Nov. 13, 2025

It was a "fallacy" and "misunderstanding of the world" if people did not "believe that the online space is as dangerous for people as the streets," he added.

From BBC • May 24, 2025

A sunk-cost fallacy rears its head, with the parties increasingly feeling they've gone too far to change course.

From Salon • Apr. 15, 2025

Had it not been exposed long ago as a fallacy?

From "1984" by George Orwell

Related Words

• deception
• falsehood
• heresy
• inconsistency
• misinterpretation
• paradox
• untruth