irrational

1 of 3

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 3

noun (1)

1

: an irrational being

2

: irrational number

irrationality

3 of 3

noun (2)

ir·​rationality "+ 

1

: the quality or state of being irrational: such as

a

: lack of being endowed with reason

b

: lack of accordance with reason : unreasonableness, foolishness

2

: something that is irrational : absurdity

3

: inequality of dispersion of different colors in refraction spectra (as between crown and flint glass)

Synonyms of irrational

Adjective

• unreasonable
• unreasoning
• illegitimate

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

Oceanfront homes satisfy both sides of the luxury equation—the rational and the irrational. —Spencer Elliott, Forbes.com, 6 June 2026

The American origin story is rooted in the notion that George III was its vanquished villain, an irrational tyrant who oppressed the American colonists. —ABC News, 3 June 2026

Noun

That’s why putting a specific time on the eight-hour round trip is just an average, which can go from the logical to the irrational in the snap of a finger. —Ahmed Walid, New York Times, 25 Mar. 2026

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

Word History

Etymology

Adjective

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

Noun (2)

Medieval Latin or New Latin irrationalitas, from Latin irrationalis irrational + -itas -ity

First Known Use

Adjective

14th century, in the meaning defined above

Noun (1)

1646, in the meaning defined at sense 1

Phrases Containing irrational

• irrational number

Rhymes for irrational

• binational
• nonrational
• 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