Definition of X noun from the Oxford Advanced American Dictionary

     

    X

     noun
    noun
    NAmE//ɛks//
     
    x(pl. Xs, X's, x's
    NAmE//ˈɛksəz//
     
    )
     
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  1. 1 [countable, uncountable] the 24th letter of the English alphabet “Xylophone” begins with (an) X/“X.”
  2. 2[uncountable] (mathematics) used to represent a number whose value is not mentioned The equation is impossible for any value of x greater than 2.
  3. 3[uncountable] a person, a number, an influence, etc. that is not known or not named Let's suppose X knows what Y is doing. see also X chromosome, X-rated, X-ray
See the Oxford Advanced Learner's Dictionary entry: X