irrational

1 of 2

adjective

ir·​ra·​tio·​nal i-ˈra-sh(ə-)nəl 

ˌi(r)-

: not rational: such as

a(1)

: lacking usual or normal mental clarity or coherence

(2)

: not endowed with reason or understanding

b

: not governed by or according to reason

irrational fears

c Greek and 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

irrationality

i-ˌra-shə-ˈna-lə-tē 

ˌi(r)-

noun

irrationally

i-ˈra-sh(ə-)nə-lē 

ˌi(r)-

adverb

irrational

2 of 2

noun

1

: an irrational being

2

: irrational number

Synonyms

Adjective

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

Examples of irrational in a Sentence

Adjective

He became irrational as the fever got worse.

She had an irrational fear of cats.

Recent Examples on the Web

Adjective

It’s been reopened in chunks, thanks to the unrelenting optimism and irrational persistence of a corps of downtown residents led by Rodriguez and Rosa Chang. —Justin Davidson, Curbed, 16 June 2025

On the other hand, especially given that the vote was still restricted to only a small minority of propertied men, the rise of party politics itself sharpened the age-old mistrust of popular judgment as irrational and easily swayed—especially by lies. —Fara Dabhoiwala, Harpers Magazine, 4 June 2025

Noun

In Eggers’s vision, desire is the prime source of the irrational. —Richard Brody, The New Yorker, 23 Dec. 2024

The book comprises portraits of healers, including W. H. R. Rivers, who treated soldiers who suffered from shell shock during the First World War, and Paul Robeson, who found solace in intuition and in the irrational. —Condé Nast, The New Yorker, 17 July 2023

Word History

Etymology

Adjective

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

First Known Use

Adjective

14th century, in the meaning defined above

Noun

1646, in the meaning defined at sense 1

Phrases Containing irrational

• irrational number

Rhymes for irrational

• binational
• subnational
• transnational
• international
• multinational
• supranational
• national
• rational

Kids Definition

irrational

adjective

ir·​ra·​tio·​nal

(ˈ)ir-ˈ(r)ash-nəl,

-ən-ᵊl

1

: unable to reason

irrational beasts

irrational from fever

2

: not based on reason

irrational fear

3

: being an irrational number

an irrational root of an equation

irrationality

(ˌ)ir-ˌ(r)ash-ə-ˈnal-ət-ē

noun

irrationally

(ˈ)ir-ˈ(r)ash-nə-lē

-ən-ᵊl-ē

adverb

Medical Definition

irrational

adjective

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

: 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

(ˌ)ir-ˌ(r)ash-ə-ˈnal-ət-ē 

noun

plural irrationalities

irrationally

(ˈ)ir-ˈ(r)ash-nə-lē, -ən-ᵊl-ē 

adverb

Legal Definition

irrational

adjective

ir·​ra·​tio·​nal ir-ˈra-shə-nəl 

: 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 

noun

irrationally adverb