irrational

adjective ir·ra·tio·nal \ i-ˈra-sh(ə-)nəl , ˌi(r)- \

Definition of irrational

:not rational: such as

a (1) :not endowed with reason or understanding

(2) :lacking usual or normal mental clarity or coherence

b :not governed by or according to reason

irrational fears

c Greek & 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

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irrationality

play \i-ˌra-shə-ˈna-lə-tē, ˌi(r)-\ noun

—

irrationally

play \i-ˈra-sh(ə-)nə-lē, ˌi(r)-\ adverb

See irrational defined for English-language learners

See irrational defined for kids

Examples of irrational in a Sentence

He became irrational as the fever got worse.
She had an irrational fear of cats.

Recent Examples of irrational from the Web

Pizzagate?), and going back to the late 20th century, when irrational fears moved slower and lasted longer, there was Satan.

— avi selk, chicagotribune.com, "Falsely accused of satanic horrors, a couple spent 21 years in prison. Now they're owed millions.," 26 Aug. 2017
The New Atheists have long harbored an irrational fear of Christianity, but Christophobia doesn’t worry the Left. Combatting Islamophobia,

— elliot kaufman, National Review, "What Ever Happened to the New Atheists?," 28 July 2017
This special report will seek to unravel the causes of this irrational enmity, and to explore the contrasting internal dynamics in both countries that sustain it.

— The Economist, "Hissing cousinsWhy India and Pakistan hate each other," 22 July 2017
An exploration of romance at its most gloriously irrational.

— miranda popkey, GQ, "Adam Sandler's Best Performance, Punch-Drunk Love, Is on Netflix Right Now," 16 July 2017
The answer is irrational fear of biological engineering.

— Time, "We Need Other Options to Fight Zika," 30 June 2017
Twenty years ago, when irrational fear of being infected with HIV/AIDS was at its height, Diana was photographed touching an HIV-positive man, and that one gesture altered public attitudes.

— angela levin, Newsweek, "Exclusive: Prince Harry on Chaos After Diana's Death and Why the World Needs 'the Magic' of the Royal Family," 21 June 2017
Part of it will require members of Congress to stop parroting the gun lobby's talking points that keep those irrational fears alive.

— liz stark, CNN, "Where does GOP baseball shooting leave the gun control debate?," 16 June 2017
Porn has always been a place for indulging irrational, secret, and socially unacceptable desires — which makes it a place where people feel free to let their racial prejudices and fantasies run wild, too.

— maureen o’connor, The Cut, "Pornhub Is the Kinsey Report of Our Time," 11 June 2017

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.

Origin and Etymology of irrational

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

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First Known Use: 14th century

See Words from the same year

irrational Synonyms

Synonyms

fallacious, illegitimate, inconsequent, inconsequential, invalid, illogical, nonrational, unreasonable, unreasoning, unsound, weak

Antonyms

logical, rational, reasonable, sound, valid, well-founded, well-grounded

Related Words

eristic (also eristical), misleading, sophistic (or sophistical), specious; half-baked, ill-advised, misguided, unconsidered, unreasoned; inconsistent; absurd, asinine, foolish, meaningless, nonsensical, preposterous, reasonless, senseless, silly; Kafkaesque, odd, peculiar, strange, surreal, unusual, weird; insane, mad, nutty, wacky (also whacky); disordered, disorganized, rambling, random; unconvincing; inexplicable, unaccountable, unexplainable

Near Antonyms

commonsense, sane, sensible, sober, wise; enlightened, informed, just, justified, reasoned; ordered, organized; clear, cogent, compelling, convincing, credible, persuasive, plausible, satisfying, solid; certain, sure, true; confirmed, corroborated, demonstrated, established, substantiated, validated

irrational

noun

Definition of irrational

1 :an irrational being

2 :irrational number

First Known Use of irrational

1646

IRRATIONAL Defined for English Language Learners

irrational

adjective

Definition of irrational for English Language Learners

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

IRRATIONAL Defined for Kids

irrational

adjective ir·ra·tio·nal \ i-ˈra-shə-nᵊl \

Definition of irrational for Students

1 :not able to reason

The fever made him irrational.

2 :not based on reason

irrational fears

—

irrationally

adverb

Medical Dictionary

irrational

adjective ir·ra·tio·nal \ (ˈ)ir-ˈ(r)ash-nəl, -ən-ᵊl \

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

—

irrationality

play \(ˌ)ir-ˌ(r)ash-ə-ˈnal-ət-ē\ noun plural -ties

—

irrationally

play \(ˈ)ir-ˈ(r)ash-nə-lē, -ən-ᵊl-ē\ adverb

Law Dictionary

irrational

adjective ir·ra·tio·nal \ ir-ˈra-shə-nəl \

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

—

irrationality

play \ir-ˌra-shə-ˈna-lə-tē\ noun

—

irrationally

adverb

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See words that rhyme with irrational Thesaurus: All synonyms and antonyms for irrational Spanish Central: Translation of irrational Nglish: Translation of irrational for Spanish speakers Britannica English: Translation of irrational for Arabic speakers

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Synonyms

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irrational

Definition of irrational

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IRRATIONAL Defined for English Language Learners

irrational

Definition of irrational for English Language Learners

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irrational

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irrational

medical Definition of irrational

irrationality

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Law Dictionary

irrational

legal Definition of irrational

irrationality

irrationally

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