Definition of: x, X

(eks) noun plural x's or X's, xs or Xs, ex·es (ek′siz)
1. The 24th letter of the English alphabet: from the ancient western Greek alphabets of Chalcis, Boeotia, and Elis, and Roman X.
2. The sound of the letter x: in English variously sounded as (ks), as in axle, box, next; (gz), as in executive, exert; (ksh), as in noxious; (gzh), as in luxurious; and initially, always (z) as in xenophobe, xylophone, Xanthippe. See ALPHABET.
—symbol
1. The Roman numeral ten. See under NUMERAL.
2. Math. The principal unknown quantity; hence, anything unknown.
3. A mark shaped like an X, representing the signature of one who cannot write.
4. A mark used in diagrams, maps, etc., to place some event or substance, or to point out something to be emphasized.
5. A symbol used to indicate a kiss.
6. Anything shaped like an X.
—X marks the spot Colloq. “Here”: used in diagrams, maps, or the like, to indicate a specific locality.