irrational

American  

[ih-rash-uh-nl, -rash-nl] / ɪˈræʃ ə nl, -ˈræʃ nl /

adjective

1. without the faculty of reason; deprived of reason.

2. without or deprived of normal mental clarity or sound judgment.

3. not in accordance with reason; utterly illogical.

   irrational arguments.

   Synonyms:

   insensate, ridiculous, unreasonable

4. not endowed with the faculty of reason.

   irrational animals.

5. Mathematics.

   1. (of a number) not capable of being expressed exactly as a ratio of two integers.

   2. (of a function) not capable of being expressed exactly as a ratio of two polynomials.

6. Algebra. (of an equation) having an unknown under a radical sign or, alternately, with a fractional exponent.

7. Greek and Latin Prosody.

   1. of or relating to a substitution in the normal metrical pattern, especially a long syllable for a short one.

   2. noting a foot or meter containing such a substitution.

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noun

1. Mathematics. irrational number.

irrational British  

/ ɪˈræʃənəl /

adjective

1. inconsistent with reason or logic; illogical; absurd

2. incapable of reasoning

3. maths

   1. not rational

   2. ( as noun )

      an irrational

4. prosody

   1. of or relating to a metrical irregularity, usually the occurrence of a long syllable instead of a short one

   2. denoting a metrical foot where such an irregularity occurs

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Other Word Forms

• irrationally
  adverb
• irrationalness
  noun
• nonirrational
  adjective
• nonirrationalness
  noun

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.

Example Sentences

John says at first the changes were attributed to the menopause, but in March 2023 he noticed his wife had become more irrational and did not react as expected when she had a car crash.

From BBC • May 6, 2026

We’re clinging to that belief but will concede that it may be irrational.

From Slate • May 2, 2026

Individual investors have been pulling money out of private-credit funds at a rising pace this year, a phenomenon fund managers have blamed on irrational fears fanned by the media.

From The Wall Street Journal • Apr. 24, 2026

Asked later about stress levels on the ground, NASA's associate administrator said "it's impossible to say you don't have any irrational fears left."

From Barron's • Apr. 10, 2026

And I knew he had done that, not out of any concern for me, but because when he wants something, he gets impatient and irrational about people who make him wait.

From "The Joy Luck Club" by Amy Tan

Related Words

• aberrant
• absurd
• crazy
• foolish
• incoherent
• insane
• preposterous
• ridiculous
• stupid
• unreasonable
• unsound
• unwise
• wrong