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
Steven Thrash, Entertainment Weekly, 24 Oct. 2025
The study, published on the research platform arXiv, found that once the models were allowed to vary their bets and set their own targets, irrational behavior surged — and bankruptcy became a common outcome.
—Jesus Mesa, MSNBC Newsweek, 22 Oct. 2025
Richard Brody, The New Yorker, 23 Dec. 2024
But riveted mankind is hopelessly addicted to the irrational, with reliably disastrous results, planetwide.
—Alison Fishburn, Longreads, 17 Sep. 2022
See All Example Sentences for irrational
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Adjective
This guy might be single-handedly responsible for generations of people having an irrational fear of clowns.
—Steven Thrash, Entertainment Weekly, 24 Oct. 2025
The study, published on the research platform arXiv, found that once the models were allowed to vary their bets and set their own targets, irrational behavior surged — and bankruptcy became a common outcome.
—Jesus Mesa, MSNBC Newsweek, 22 Oct. 2025
Noun
In Eggers’s vision, desire is the prime source of the irrational.
—Richard Brody, The New Yorker, 23 Dec. 2024
But riveted mankind is hopelessly addicted to the irrational, with reliably disastrous results, planetwide.
—Alison Fishburn, Longreads, 17 Sep. 2022
See All Example Sentences for irrational
Cite this Entry
“Irrational.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/irrational. Accessed 31 Oct. 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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