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See more synonyms for irrational on Thesaurus.com
  1. without the faculty of reason; deprived of reason.
  2. without or deprived of normal mental clarity or sound judgment.
  3. not in accordance with reason; utterly illogical: irrational arguments.
  4. not endowed with the faculty of reason: irrational animals.
  5. Mathematics.
    1. (of a number) not capable of being expressed exactly as a ratio of two integers.
    2. (of a function) not capable of being expressed exactly as a ratio of two polynomials.
  6. Algebra. (of an equation) having an unknown under a radical sign or, alternately, with a fractional exponent.
  7. Greek and Latin Prosody.
    1. of or relating to a substitution in the normal metrical pattern, especially a long syllable for a short one.
    2. noting a foot or meter containing such a substitution.
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  1. Mathematics. irrational number.
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Origin of irrational

First recorded in 1425–75; late Middle English word from Latin word irratiōnālis. See ir-2, rational
Related formsir·ra·tion·al·ly, adverbir·ra·tion·al·ness, nounnon·ir·ra·tion·al, adjective, nounnon·ir·ra·tion·al·ly, adverbnon·ir·ra·tion·al·ness, noun

Synonyms for irrational

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Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Related Words for irrational

insane, crazy, stupid, incoherent, unreasonable, preposterous, unsound, wrong, ridiculous, absurd, unwise, foolish, aberrant, wild, loony, raving, silly, invalid, brainless, delirious

Examples from the Web for irrational

Contemporary Examples of irrational

Historical Examples of irrational

  • I haven't been irrational, I give you my word, not once since I came here.

    The Spenders

    Harry Leon Wilson

  • This reply seemed to arouse an irrational anger in the Briton.

  • If men cannot have a rational belief, they will have an irrational.



  • Pride was irrational, and so was the greater part of human misery.

    Mezzerow Loves Company

    Floyd L. Wallace

  • In spite of his fine manner and bearing, his every word had been irrational.

British Dictionary definitions for irrational


  1. inconsistent with reason or logic; illogical; absurd
  2. incapable of reasoning
  3. maths
    1. not rational
    2. (as noun)an irrational
  4. prosody (in Greek or Latin verse)
    1. of or relating to a metrical irregularity, usually the occurrence of a long syllable instead of a short one
    2. denoting a metrical foot where such an irregularity occurs
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Derived Formsirrationally, adverbirrationalness, noun
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

Word Origin and History for irrational


late 15c., "not endowed with reason" (of beats, etc.); earlier (of quantities) "inexpressible in ordinary numbers" (late 14c.); from Latin irrationalis "without reason," from assimilated form of in- "not, opposite of" (see in- (1)) + rationalis "reason" (see rational). Meaning "illogical, absurd" is attested from 1640s. Related: Irrationally.

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Online Etymology Dictionary, © 2010 Douglas Harper

irrational in Medicine


  1. Not rational; marked by a lack of accord with reason or sound judgment.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.