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
1
: an irrational being
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
Forrester, Forbes.com, 2 July 2025
As Reva relates the stories of her three main characters—including one whose true passion is snail conservation—her novel hums with bruised faith in the irrational power of hope, whether for peace, love, endangered species, or familial reconciliation.
—The New Yorker, New Yorker, 23 June 2025
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
See All Example Sentences for irrational
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Adjective
So watch for seemingly irrational behavior and erratic tactical moves, as leadership is unafraid to confuse customers and take outsized capital risks.
—Forrester, Forbes.com, 2 July 2025
As Reva relates the stories of her three main characters—including one whose true passion is snail conservation—her novel hums with bruised faith in the irrational power of hope, whether for peace, love, endangered species, or familial reconciliation.
—The New Yorker, New Yorker, 23 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
See All Example Sentences for irrational
Cite this Entry
“Irrational.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/irrational. Accessed 18 Jul. 2025.
Copy CitationMedical 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
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
irrationally
adverb
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