- 1 [countable, uncountable] the 24th letter of the English alphabet “Xylophone” begins with (an) X/“X.”
- 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[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
NAmE//ɛks//x (pl. Xs, X's, x's