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; ir- ² ( def. ), rational ( def. )

Example Sentences

While the motives that drive a knight are often human, political, or both, the logic is as miraculous and irrational as a muddy hedge knight crawling out from under a fallen dragon.

From Salon • Feb. 25, 2026

Michael Taylor, chair of the Friends of Brockwell Park, told the meeting: "It is irrational to conclude that there is only limited temporary harm in the delicate heritage balance."

From BBC • Feb. 24, 2026

This was an irrational fear, but this was where Shiffrin was at this point.

From The Wall Street Journal • Feb. 18, 2026

Freight was another sector that some analysts said was subjected to irrational AI fears last week.

From MarketWatch • Feb. 17, 2026

To have positive liberty, he explained, is to take control of one’s own mind; to be liberated from irrational fears and beliefs, from addictions, superstitions and all other forms of self-coercion.

From "Educated" by Tara Westover

Related Words

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