irrational
Americanadjective
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without the faculty of reason; deprived of reason.
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without or deprived of normal mental clarity or sound judgment.
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not in accordance with reason; utterly illogical.
irrational arguments.
- Synonyms:
- insensate, ridiculous, unreasonable
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not endowed with the faculty of reason.
irrational animals.
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Mathematics.
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(of a number) not capable of being expressed exactly as a ratio of two integers.
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(of a function) not capable of being expressed exactly as a ratio of two polynomials.
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Algebra. (of an equation) having an unknown under a radical sign or, alternately, with a fractional exponent.
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Greek and Latin Prosody.
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of or relating to a substitution in the normal metrical pattern, especially a long syllable for a short one.
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noting a foot or meter containing such a substitution.
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noun
adjective
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inconsistent with reason or logic; illogical; absurd
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incapable of reasoning
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maths
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not rational
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( as noun )
an irrational
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prosody
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of or relating to a metrical irregularity, usually the occurrence of a long syllable instead of a short one
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denoting a metrical foot where such an irregularity occurs
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Other Word Forms
Derived Forms
Etymology
Origin of irrational
First recorded in 1425–75; late Middle English, from Latin irratiōnālis; see ir- 2 ( def. ), rational ( def. )
Explanation
If you're a straight-A student and still you worry about failing all of your classes, you're being irrational. Your fears are not based on fact and not likely to come true. Usually you use irrational to describe ideas and thoughts that are not based on reason, although emotions and behavior can be irrational too if they don't seem reasonable. Do you jump onto a chair and scream when you see a mouse? If so, you have an irrational fear of mice, or musophobia. If someone takes a swing at you for failing to say excuse me when you burp, both their anger and their actions could be called irrational.
Vocabulary lists containing irrational
Pi Day Vocabulary
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The Number System
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Divergent
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
Macgregor had never known her old friend to be irrational.
From The Wall Street Journal • May 23, 2026
“The current ‘flash-sale wars’ are typical competition under demand-side economies of scale but have escalated into irrational capital burn,” said Chen Tianhao, an associate professor at Tsinghua University.
From Barron's • May 23, 2026
“I think that irrational exuberance that we saw in the dot com era … is just window dressing to cloak what, to me, is normalizing an addictive behavior in our society,” he said.
From MarketWatch • May 19, 2026
She did some moves that were pretty irrational, and she would have times where she would act on impulse, and it became more and more unstable and unsafe.
From Los Angeles Times • May 13, 2026
The terrifying irrational has no place in classical mythology.
From "Mythology: Timeless Tales of Gods and Heroes" by Edith Hamilton
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
