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
• error
• falsehood
• falsity
• hallucination
• illusion
• misbelief
• misconception
• myth
• old wives' tale
• untruth

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

One such skill is identifying logical fallacies and inconsistencies in arguments, both of which errors are exemplified in Mr. Williams’ article. —Reader Commentary, Baltimore Sun, 7 May 2025

That fallacy is now deeply entrenched in the U.S. mindset, thanks to generations of leaders who believed that diplomacy would create a liberal utopia. —A. Wess Mitchell, Foreign Affairs, 22 Apr. 2025

To understand the financial fallacy behind the idea that $5,000 is a reasonable return on the investment required to raise a child, just ask any parent their thoughts. —Aisha Nyandoro, Forbes.com, 30 Apr. 2025

However, between The New Adventures of Old Christine and Veep, Louis-Dreyfus would go on to squash that fallacy to the tune of ten more Emmy wins. —Brian Davids, HollywoodReporter, 29 Apr. 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