: not rational: such as
a
: not thinking clearly : not able to use reason or good judgment
He became irrational as the fever got worse.
b
: not based on reason, good judgment, or clear thinking
She had an irrational fear of cats.
an irrational prejudice
Cite this Entry
“Irrational.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/irrational. Accessed 20 Jun. 2026.
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