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 And whatever their reasoning, however irrational, a handful of players are standing by their convictions., 2 Oct. 2021 The goal of combining all these disciplines is to ultimately learn about the mental, emotional, irrational, and behavioral nuances governing our buying habits. Prince Ghuman, Forbes, 2 Oct. 2021 This duality makes her character -- and the movie -- all the more challenging, as we're asked to grapple with the rational and the irrational, the acceptable and the otherworldly. CNN, 17 Sep. 2021 In place of Mackay’s intuitive insights into crowd madness, Bernstein draws on research in the field of behavioral psychology to distinguish between rational and irrational economic behavior. Edward Chancellor, The New York Review of Books, 19 Aug. 2021 Yet historians know that American drug laws are anything but irrational, arbitrary or absurd. Washington Post, 23 July 2021 The status-quo managers are not being entirely irrational. Steve Denning, Forbes, 13 Sep. 2021 The era of irrational exuberance was already running on fumes by the time the Twin Towers fell. Cynthia Littleton, Variety, 27 Aug. 2021 You may be tempted to display irrational or unnecessarily mean behavior due to your own internal conflict. Meghan Ros, Glamour, 1 Sep. 2021 Recent Examples on the Web: Noun But feeling anxious right now is not at all out of line or irrational. Jessica Dulong, CNN, 31 Aug. 2021 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, 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, 29 Apr. 2021 We are all steered by a mixture of the rational and the irrational. Christopher Beha, Harpers Magazine, 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, 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, 16 Sep. 2019 Instead, mathematicians ask about the proportion of the total number of irrationals corralled by each fraction. Quanta Magazine, 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

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

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Last Updated

14 Oct 2021

Cite this Entry

“Irrational.” Dictionary, Merriam-Webster, Accessed 18 Oct. 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

More from Merriam-Webster on irrational

Nglish: Translation of irrational for Spanish Speakers

Britannica English: Translation of irrational for Arabic Speakers


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