Definition of irrational in English:



  • 1Not logical or reasonable.

    ‘irrational feelings of hostility’
    • ‘Identify and uproot irrational beliefs that lead you to place unrealistic demands on yourself.’
    • ‘He had a very short fuse and he could be totally irrational at times.’
    • ‘Australians aren't at all used to visceral and irrational hatred directed at them.’
    • ‘But Delta's seemingly irrational behavior has a very logical explanation.’
    • ‘By absurd, I do not mean silly, but absurd as is in the illogical or irrational.’
    • ‘The more irrational and illogical someone becomes, the more logical and incisive I become.’
    • ‘The author of the piece argues that our attitude towards it is mostly irrational prejudice.’
    • ‘Their hostility towards each other, however, was tangible and frequently led to quite irrational behaviour.’
    • ‘Without that insight, a supplier can encounter fewer surprises by evaluating even seemingly irrational scenarios.’
    • ‘Home values have soared to high levels in many countries as irrational exuberance grips the markets.’
    • ‘Chris smiled, sure that Seth's fears were completely irrational.’
    • ‘If it hadn't seemed so irrational, she would have begged him not to go.’
    • ‘Solutions that would seem logical to us would seem totally irrational to them.’
    • ‘Power sets the guidelines by which we measure what is reasonable and what is irrational.’
    • ‘I believe that the seemingly irrational logic of the national can still promote everyday alliances and popular mobilizations.’
    • ‘I have an irrational dislike of cell phones.’
    • ‘Booze and cocaine corroded his sanity and left him with a legacy of irrational behaviour.’
    • ‘She had never known for her daughter to be so irrational, so crazy.’
    • ‘I am sort of notorious for my utterly irrational fears.’
    • ‘Crowds and mobs are not completely irrational, but they have their own logic.’
    unreasonable, illogical, groundless, baseless, unfounded, unjustifiable, unsound
    absurd, ridiculous, ludicrous, silly, foolish, senseless, nonsensical, laughable, idiotic, stupid, wild
    untenable, implausible, unscientific, arbitrary
    crazy, mad
    barmy, daft
    View synonyms
    1. 1.1Not endowed with the power of reason.
      • ‘Man is an irrational being, morals are irrational, and have no metaphysical foundation which make them "real" or worth paying any attention to.’
      • ‘Whenever you encounter a system that seems so irrational, you should ponder what's going on beneath the surface.’
  • 2Mathematics
    (of a number, quantity, or expression) not expressible as a ratio of two integers, and having an infinite and non-recurring expansion when expressed as a decimal. Examples of irrational numbers are the number π and the square root of 2.

    • ‘A transcendental number is an irrational number that is not a root of any polynomial equation with integer coefficients.’
    • ‘The square root of 2 is an irrational number because it can't be written as a ratio of two integers.’
    • ‘He considered computation with irrational numbers and polynomials to be part of algebra.’
    • ‘How can mathematical concepts like points, infinitesimally small quantities, or irrational numbers be anything but products of our minds?’
    • ‘What about a seed angle derived from the golden ratio, an irrational number?’


  • An irrational number or quantity; a surd.

    • ‘His commentary to Euclid is of interest because of its discussion of unordered irrationals.’
    • ‘Whether such quirks in the irregularity of irrationals have any implications for number theory remains an open question for mathematicians.’
    • ‘Any finite segment can be continued to produce a rational and any finite segment can be continued to produce an irrational.’
    • ‘If we chop off an infinite cfe after a finite number of steps then we will create a rational approximation to the original irrational.’
    • ‘Eudoxus's definition of equal ratios corresponds exactly to the modern theory of irrationals.’


Late Middle English: from Latin irrationalis, from in- not + rationalis (see rational).