X

X 24th letter of the alphabet . In English it has no peculiar sound, but stands for the combination ks as in fox, or gz as in exempt, or, initially, for the sound of z as in xenia. In words from Greek, x transliterates the 14th Greek letter, xi, a letter perhaps quite unrelated to the Roman x in form. The formal Greek correspondent of x was chi, hence it is used in phonetics to represent a velar fricative like ch in loch. As the initial of the name Christ, X has become a symbol for it, e.g., in Xmas and in the monogram XP (chi rho). In Roman numerals X stands for 10.

X¹ / eks/ (also x) • n. (pl. Xs or X's ) 1. the twenty-fourth letter of the alphabet. ∎  denoting the next after W in a set of items, categories, etc. ∎  denoting an unknown or unspecified person or thing: there is nothing in the data to tell us whether X causes Y. ∎  (x) (used in describing play in bridge) denoting an unspecified card other than an honor. ∎  (usu. x) the first unknown quantity in an algebraic expression, usually the independent variable. ∎  (usu. x) denoting the principal or horizontal axis in a system of coordinates: [in comb.] the x-axis. 2. a cross-shaped written symbol, in particular: ∎  used to indicate a position on a map or diagram. ∎  used to indicate a mistake or incorrect answer. ∎  used in a letter or message to symbolize a kiss. ∎  used to indicate one's vote on a paper ballot. ∎  used in place of the signature of a person who cannot write. 3. a shape like that of a letter X: two wires in the form of an X | [in comb.] an X-shaped cross. 4. the Roman numeral for ten. • v. (X’s, X’d, X’ing) [tr.] mark or make a sign with an X. ∎  overwrite or obliterate with an X or series of X’s. ∎  make void or annul; invalidate: we’re all X-ing things out of our curricula. X² • symb. 1. a rating assigned to movies classified as suitable for adults only. Replaced in 1990 by NC-17. 2. (in systematic names of organisms) hybrid.