fallacy

noun

fal·​la·​cy ˈfa-lə-sē 

plural fallacies

1

a

: a false or mistaken idea

popular fallacies

prone to perpetrate the fallacy of equating threat with capability

—C. S. Gray

b

: erroneous character : erroneousness

The fallacy of their ideas about medicine soon became apparent.

2

a

: deceptive appearance : deception

b

obsolete : guile, trickery

3

: an often plausible argument using false or invalid inference

Did you know?

Philosophers are constantly using the word fallacy. For them, a fallacy is reasoning that comes to a conclusion without the evidence to support it. This may have to do with pure logic, with the assumptions that the argument is based on, or with the way words are used, especially if they don't keep exactly the same meaning throughout the argument. There are many classic fallacies that occur again and again through the centuries and everywhere in the world. You may have heard of such fallacies as the "ad hominem" fallacy, the "question-begging" fallacy, the "straw man" fallacy, the "slippery slope" fallacy, the "gambler's" fallacy, or the "red herring" fallacy. Look them up and see if you've ever been guilty of any of them.

Synonyms

• delusion
• myth
• error
• illusion

Examples of fallacy in a Sentence

The fallacy of their ideas about medicine soon became apparent.

the once-common fallacy that girls just weren't any good at math

Recent Examples on the Web

The list of projects that fall prey to the sunk cost fallacy are legion. —Neil Senturia, San Diego Union-Tribune, 16 June 2025

And the fallacy is that trade deficits actually mean anything. —Isabel Danzis, ABC News, 18 May 2025

To learn more about the facts behind his fallacies, here’s a list of 100 separate false claims Trump has made since his inauguration. —Jade Walker, CNN Money, 30 Apr. 2025

Paroma Chatterjee, the country’s chief executive of Revolut, an online banking company that started in Britain, said that Covid had showed the fallacy of tethering a job to a place. —Alex Travelli, New York Times, 26 Mar. 2025

Word History

Etymology

Latin fallacia, from fallac-, fallax deceitful, from fallere to deceive

First Known Use

15th century, in the meaning defined at sense 2b

Phrases Containing fallacy

• pathetic fallacy

Kids Definition

fallacy

noun

fal·​la·​cy ˈfal-ə-sē 

plural fallacies

1

: a false or mistaken idea

2

: the quality or state of being false