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

The irrational nature of the conflict has been the main reason why it could never be resolved. —Tom Segev, Foreign Affairs, 23 Apr. 2024

The boy’s interests, at least as selected for ironic reference later, are in the predictability of decision making, under rational and even irrational circumstances. —Jesse Green, New York Times, 22 Apr. 2024

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

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