noun, plural fal·la·cies.

  1. a deceptive, misleading, or false notion, belief, etc.: That the world is flat was at one time a popular fallacy.
  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.

noun plural -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

late 15c., “deception, false statement,” from Latin fallacia “deception,” noun of quality from fallax (genitive fallacis) “deceptive,” from fallere “deceive” (see fail (v.)). Specific sense in logic dates from 1550s. An earlier form was fallace (c.1300), from Old French fallace.

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.)

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