[ ih-rash-uh-nl, -rash-nl ]
See synonyms for: irrationalirrationally on

  1. without the faculty of reason; deprived of reason.

  2. without or deprived of normal mental clarity or sound judgment.

  1. not in accordance with reason; utterly illogical: irrational arguments.

  2. not endowed with the faculty of reason: irrational animals.

  3. Mathematics.

    • (of a number) not capable of being expressed exactly as a ratio of two integers.

    • (of a function) not capable of being expressed exactly as a ratio of two polynomials.

  4. Algebra. (of an equation) having an unknown under a radical sign or, alternately, with a fractional exponent.

  5. Greek and Latin Prosody.

    • of or relating to a substitution in the normal metrical pattern, especially a long syllable for a short one.

    • noting a foot or meter containing such a substitution.

  1. Mathematics. irrational number.

Origin of irrational

First recorded in 1425–75; late Middle English, from Latin irratiōnālis; see ir-2, rational

Other words for irrational

Other words from irrational

  • ir·ra·tion·al·ly, adverb
  • ir·ra·tion·al·ness, noun
  • non·ir·ra·tion·al, adjective, noun
  • non·ir·ra·tion·al·ness, noun

Words Nearby irrational Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

How to use irrational in a sentence

British Dictionary definitions for irrational


/ (ɪˈræʃənəl) /

  1. inconsistent with reason or logic; illogical; absurd

  2. incapable of reasoning

  1. maths

    • not rational

    • (as noun): an irrational

  2. prosody (in Greek or Latin verse)

    • of or relating to a metrical irregularity, usually the occurrence of a long syllable instead of a short one

    • denoting a metrical foot where such an irregularity occurs

Derived forms of irrational

  • irrationally, adverb
  • irrationalness, noun

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012