Merriam-Webster Logo
SINCE 1828
Menu
×

An Encyclopædia Britannica Company

See What's Trending Now See More Trending Words
1

irrational

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

Definition of irrational

  1. :  not rational: such asa (1) :  not endowed with reason or understanding (2) :  lacking usual or normal mental clarity or coherenceb :  not governed by or according to reason irrational fearsc Greek & Latin prosody (1) of a syllable :  having a quantity other than that required by the meter (2) of a foot :  containing such a syllabled (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

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

Read the Latest from M-W

image1458426927
Good Words for Strong Flavors
time-traveler-quiz-which-word-came-first
Time Traveler Quiz: Which Word Came First?
alt-5991e113c97b3
Trending: Trump Calls Racism 'Repugnant'
video-title-words-of-the-year-1066
Words of the Year: 1066

Examples of irrational in a Sentence

  1. He became irrational as the fever got worse.

  2. She had an irrational fear of cats.

Origin and Etymology of irrational

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

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
2

irrational

noun ir·ra·tio·nal

Definition of irrational

  1. 1 :  an irrational being

  2. 2 :  irrational number

First Known Use of irrational

1646

IRRATIONAL Defined for English Language Learners

irrational

play
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

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

Definition of irrational for Students

  1. 1 :  not able to reason The fever made him irrational.

  2. 2 :  not based on reason irrational fears

irrationally

adverb

Medical Dictionary

irrational

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

Medical Definition of irrational

  1. :  not rational: asa:  lacking usual or normal mental clarity or coherence was irrational for several days after the accidentb:  not governed by or according to reason irrational fears

irrationality

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

irrationalities

irrationally

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

Law Dictionary

irrational

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

Legal Definition of irrational

  1. :  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

\ir-ˌra-shə-ˈna-lə-tē\ play noun

irrationally

adverb

Learn More about irrational

  1. 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

Seen and Heard

What made you want to look up irrational? Please tell us where you read or heard it (including the quote, if possible).

show hide

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!

WORD OF THE DAY

oppugn

play

to fight against or to call in question

Get Word of the Day daily email!

TRENDING NOW

  1. repugnant Trump clarifies his position on racism

  2. white nationalist, white supremacist Trending in 2017

  3. bereft "Lacking something needed, wanted, or expected"

  4. impanel "To choose people to serve on a jury"

  5. cosmopolitan "Worldly"

SEE ALL

BROWSE DICTIONARY

irrational

Test Your Vocabulary

Time Traveler Quiz: Which Word Came First?

  • time-traveler-quiz-which-word-came-first
  • Which came first?
True or False

Test your knowledge - and maybe learn something along the way.

TAKE THE QUIZ
Word Winder's CrossWinder

Test Your Knowledge - and learn some interesting things along the way.

TAKE THE QUIZ

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!

Words at Play

alt-599492594f435
How 'Wonderful' Lost Its Sense of Wonder

A case of semantic bleaching

dog-covering-face
Don't Let Reading This Be Another 'Coulda' 'Shoulda' 'Woulda'

PSA: They're three separate words.

image1458426927
Food Words for Strong Flavors

A tasting menu of vivid flavor descriptions

alt-5993066a3afad
What's the Plural of 'Maitre d''?

Use this word to confuse a French speaker.

Ask the Editors

video-title-words-of-the-year-1066
Words of the Year: 1066

Or, Why Pig Meat is Called 'Pork' and Cow Meat is Called 'Beef'

payday
How Often Is 'Biweekly'?

Don't get us started on 'bimonthly'

video-irregardless-grammar-peeve-blend-of-the-synonyms-irrespective-and-regardless
Irregardless

It is in fact a real word (but that doesn't mean you should use it).

video-moose-goose-weird-plurals
Weird Plurals

One goose, two geese. One moose, two... moose. What's up with that?

Word Games

time-traveler-quiz-which-word-came-first
Time Traveler Quiz: Which Word Came First?

Which word entered the English language first?

Take the quiz
name-more-food
Name More Food!

Test your knowledge of food vocabulary!

Take the quiz
True or False
True or False?

Test your knowledge - and maybe learn something along the way.

Take the quiz
Word Winder's CrossWinder
Word Winder's CrossWinder

A game of winding words.

Play the game
Examples: irrational in a Sentence