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Synonyms

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.

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

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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.)

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

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

“A major fallacy is assuming that policies that work in one country will automatically work in another,” Josh Michaud, the associate director of global health and public health policy at KFF, told Salon.

From Salon • Jan. 11, 2026

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

And as long as we believe in this fallacy of separation, that’s going to continue.”

From Los Angeles Times • Aug. 4, 2025

Doing so is sometimes termed the “broad base” fallacy, and we’ve already cited a couple of instances of it.

From "Innumeracy: Mathematical Illiteracy and Its Consequences" by John Allen Paulos

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.