noun, plural fallacies.
a deceptive, misleading, or false notion, belief, etc.: That the world is flat was at one time a popular fallacy.
a misleading or unsound argument.
deceptive, misleading, or false nature; erroneousness.
Logic. any of various types of erroneous reasoning that render arguments logically unsound.
Obsolete, deception.

1350–1400; < Latin fallācia a trick, deceit, equivalent to fallāc- (stem of fallāx) deceitful, fallacious + -ia -y3; replacing Middle English fallace < Middle French

1. misconception, delusion, misapprehension.
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World English Dictionary
fallacy (ˈfæləsɪ)
n , pl -cies
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
[C15: from Latin fallācia, from fallax deceitful, from fallere to deceive]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

late 15c., "deception, false statement," from L. fallacia "deception," from fallax (gen. fallacis) "deceptive," from fallere "deceive." Specific sense in logic dates from 1550s.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Cultural Dictionary

fallacy definition

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.)

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
In any case pointing out the use of a fallacy is not a fallacy.
One might call it the new biographical fallacy, born of this age of too much
The trap is the naturalistic fallacy of ethics, which uncritically concludes
  that what is, should be.
But rather than marshaling logically sound arguments, he constantly commits the
  fallacy of begging the question.
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