fallacy

American  

[fal-uh-see] / ˈfæl ə si /

noun

fallacies plural

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

Loss aversion, sunk-cost fallacy and herd behavior — all of it kicks in during a drawdown.

From MarketWatch • May 12, 2026

One tenacious fallacy about Shakespeare is that his speeches are always clotted with gargantuan words; Macbeth’s “multitudinous seas incarnadine” is a classic example.

From The Wall Street Journal • Apr. 22, 2026

"I think the thing young founders get wrong is they view hours worked in and of itself as necessary and sufficient to think of themselves as productive. And that's where the fallacy lies", he explains.

From BBC • Feb. 8, 2026

The promise is that we can shape behavior; the fallacy is that the power rarely reaches beyond cosmetic concessions.

From Salon • Aug. 28, 2025

The misconception is so common that it has been given a name: the etymological fallacy.

From "The Sense of Style" by Steven Pinker

Related Words

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