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[ih-rash-uh-nl] /ɪˈræʃ ə nl/
without the faculty of reason; deprived of reason.
without or deprived of normal mental clarity or sound judgment.
not in accordance with reason; utterly illogical:
irrational arguments.
not endowed with the faculty of reason:
irrational animals.
  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.
Algebra. (of an equation) having an unknown under a radical sign or, alternately, with a fractional exponent.
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.
Mathematics, irrational number.
late Middle English
1425-75; late Middle English < Latin irratiōnālis. See ir-2, rational
Related forms
irrationally, adverb
irrationalness, noun
nonirrational, adjective, noun
nonirrationally, adverb
nonirrationalness, noun
3. unreasonable, ridiculous; insensate. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the web for irrationally
  • When real infestations occur, even sensible people often behave irrationally.
  • Some claim that he is irrationally jealous and controlling of her, and even keeps her locked in his apartment when he is away.
  • It's a profound lesson for designers and people who irrationally fear brain implants.
  • Prudence should stop you from behaving irrationally, not from investing at all.
  • And he may feel unrelenting guilt, however irrationally.
  • It's essential to be brash and irrationally exuberant to start a business.
  • Well, one side says that people were irrationally optimistic.
  • But, for many people, opposition to the mines' reopening became irrationally bound up with the future of snowmobiling.
  • Those who live in fear of such a tragedy are engaging in magical thought and behaving irrationally.
  • We have a lot of understanding of how sentient life-forms can be made to behave irrationally.
British Dictionary definitions for irrationally


inconsistent with reason or logic; illogical; absurd
incapable of reasoning
  1. not rational
  2. (as noun): an irrational
(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
Derived Forms
irrationally, adverb
irrationalness, 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
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Word Origin and History for irrationally



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.

Online Etymology Dictionary, © 2010 Douglas Harper
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irrationally in Medicine

irrational ir·ra·tion·al (ĭ-rāsh'ə-nəl)
Not rational; marked by a lack of accord with reason or sound judgment.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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