ir·​ra·​tio·​nal | \ i-ˈra-sh(ə-)nəl How to pronounce irrational (audio) , ˌi(r)- \

Definition of irrational

 (Entry 1 of 2)

: not rational: such as
a(1) : lacking usual or normal mental clarity or coherence
(2) : not endowed with reason or understanding
b : not governed by or according to reason irrational fears
c Greek and Latin prosody
(1) of a syllable : having a quantity other than that required by the meter
(2) of a foot : containing such a syllable
d(1) : being an irrational number an irrational root of an equation
(2) : having a numerical value that is an irrational number a length that is irrational



Definition of irrational (Entry 2 of 2)

1 : an irrational being

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Other Words from irrational


irrationality \ i-​ˌra-​shə-​ˈna-​lə-​tē How to pronounce irrational (audio) , ˌi(r)-​ \ noun
irrationally \ i-​ˈra-​sh(ə-​)nə-​lē How to pronounce irrational (audio) , ˌi(r)-​ \ adverb

Examples of irrational in a Sentence

Adjective He became irrational as the fever got worse. She had an irrational fear of cats.
Recent Examples on the Web: Adjective In the absence of good evidence recommending one treatment over another, trying to stamp out treatment variation is irrational. Sandeep Jauhar, WSJ, "The Crushing Burden of Healthcare Microregulation," 28 Apr. 2021 Many of them have become proficient at hiding their symptoms, which can include irrational fears, an overly self-conscious presentation, fear of humiliation or embarrassment, and excessive worry around social interactions. John Duffy, CNN, "These people thrived in pandemic isolation -- and aren't ready to return to 'normal' socializing," 9 Apr. 2021 These are movies documenting the dramatic ups and downs of business phenomena driven by compelling figures who use their charms to lead investors and employees to irrational behavior and groupthink. Ryan Faughnder, Los Angeles Times, "Hollywood is obsessed with ‘cult-like’ business leaders. WeWork is no exception," 6 Apr. 2021 The way to sell these bad products is to tap into investors’ irrational fears and push conspiracy theories. Michael Taylor, San Antonio Express-News, "Taylor: Thankfully, is out of business, but the precious-metal investment scourge remains," 25 Mar. 2021 And so are our Italian mythological capitalists: full of energy, of imagination, of relentless impulses that often lead to irrational personal behavior, given that that mythological theory is primitive. Anna Zanardi Cappon, Forbes, "Ring Around The Rosie, A Pocketful Of Posies. A-Tishoo!, A-Tishoo! We All Fall Down!," 12 Mar. 2021 But some worry whether this the investing social media space, and the obsession with finance influencers and star managers, could lead to irrational herd investment decisions. Jane Li, Quartz, "China’s Gen Z investors are turning fund managers into social media stars," 12 Mar. 2021 Lots of other technologies—genetic engineering, high-temperature superconductors, nanotechnology and fusion energy come to mind—have gone through phases of irrational exuberance. John Horgan, Scientific American, "Will Quantum Computing Ever Live Up to Its Hype?," 20 Apr. 2021 NFTs have provoked plenty of irrational exuberance, but there is no denying one aspect of their nature: Their authenticity. Michael Spellacy, Forbes, "Authentic Opportunity: Wealth Managers Need A Strategy For Digital Assets," 12 Apr. 2021 Recent Examples on the Web: Noun Attempting to appease the loudest to the detriment of the community only emboldens the irrational and silences those looking to have productive conversation. Benjamin Ayanian, Star Tribune, "Radical rhetoric on U policing is irresponsible," 5 May 2021 To believe in the return of a long-dead child reflects the anguish of the believer and, of course, a tendency to embrace the irrational. Dorothy Rabinowitz, WSJ, "‘The Drowning’ Review: When the Past Resurfaces," 29 Apr. 2021 We are all steered by a mixture of the rational and the irrational. Christopher Beha, Harpers Magazine, "A Novel Approach," 5 Jan. 2021 In the battle between the infinitely large and the infinitely small, Dirichlet had to find the right balance to prevent some irrationals from slipping through the cracks. Quanta Magazine, "How Rational Math Catches Slippery Irrational Numbers," 10 Mar. 2020 These unwieldy numbers have plagued mathematicians since the ancient Greeks; indeed, legend has it that Hippasus was drowned for suggesting irrationals existed. Leila Sloman, Scientific American, "New Proof Solves 80-Year-Old Irrational Number Problem," 16 Sep. 2019 Instead, mathematicians ask about the proportion of the total number of irrationals corralled by each fraction. Quanta Magazine, "New Proof Settles How to Approximate Numbers Like Pi," 14 Aug. 2019

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'irrational.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of irrational


14th century, in the meaning defined above


1646, in the meaning defined at sense 1

History and Etymology for irrational


Middle English, from Latin irrationalis, from in- + rationalis rational

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Time Traveler for irrational

Time Traveler

The first known use of irrational was in the 14th century

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Statistics for irrational

Last Updated

11 May 2021

Cite this Entry

“Irrational.” Dictionary, Merriam-Webster, Accessed 13 May. 2021.

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More Definitions for irrational



English Language Learners Definition of irrational

: not thinking clearly : not able to use reason or good judgment
: not based on reason, good judgment, or clear thinking


ir·​ra·​tio·​nal | \ i-ˈra-shə-nᵊl How to pronounce irrational (audio) \

Kids Definition of irrational

1 : not able to reason The fever made him irrational.
2 : not based on reason irrational fears

Other Words from irrational

irrationally adverb


ir·​ra·​tio·​nal | \ (ˈ)ir-ˈ(r)ash-nəl, -ən-ᵊl How to pronounce irrational (audio) \

Medical Definition of irrational

: not rational: as
a : lacking usual or normal mental clarity or coherence was irrational for several days after the accident
b : not governed by or according to reason irrational fears

Other Words from irrational

irrationality \ (ˌ)ir-​ˌ(r)ash-​ə-​ˈnal-​ət-​ē How to pronounce irrational (audio) \ noun, plural irrationalities
irrationally \ (ˈ)ir-​ˈ(r)ash-​nə-​lē, -​ən-​ᵊl-​ē How to pronounce irrational (audio) \ adverb

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ir·​ra·​tio·​nal | \ ir-ˈra-shə-nəl How to pronounce irrational (audio) \

Legal Definition of irrational

: not rational: as
a : not governed by reason, mental clarity, or understanding
b : not governed by a fair consideration of facts or evidence broadly : arbitrary an irrational decision to deny the permit

Other Words from irrational

irrationality \ ir-​ˌra-​shə-​ˈna-​lə-​tē How to pronounce irrational (audio) \ noun
irrationally adverb

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