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

Adapt And Act With Integrity The ability to foresee every PR crisis is a fallacy. —Expert Panel®, Forbes.com, 8 July 2025

Denialists tend to cherry-pick evidence, set impossible levels of evidence and engage in logical fallacies. —H. Christopher Frey, The Conversation, 5 June 2025

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

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