X, x

noun, plural X's or Xs, x's or xs.
  1. the 24th letter of the English alphabet, a consonant.
  2. any spoken sound or combination of sounds represented by the letter X or x, as in xylene, box, or exact.
  3. something having the shape of an X.
  4. a written or printed representation of the letter X or x.
  5. a device, as a printer's type, for reproducing the letter X or x.


verb (used with object), x-ed or x'd [ekst] /ɛkst/, x-ing or x'ing [ek-sing] /ˈɛk sɪŋ/.
  1. to cross out or mark with or as if with an x (often followed by out): to x out an error.
  2. to indicate choice, as on a ballot or examination (often followed by in): to x in the candidate of your choice.

Origin of x

First recorded in 1840–50


  1. ex1(def 1).
  2. excess.
  3. Stock Exchange.
    1. (of stock trading) ex dividend.
    2. (of bond trading) ex interest.
  4. experimental.
  5. extra.


  1. an unknown quantity or a variable.
  2. (used at the end of letters, telegrams, etc., to indicate a kiss.)
  3. (used to indicate multiplication) times: 8 × 8=64.
  4. (used between figures indicating dimensions) by: 3″ × 4″ (read: “three by four inches”); 3″ × 4″ × 5″ (read: “three by four by five inches”).
  5. power of magnification: a 50x telescope.
  6. (used as a signature by an illiterate person.)
  7. cross.
  8. crossed with.
  9. (used to indicate a particular place or point on a map or diagram.)
  10. out of; foaled by: a colt by Flag-a-way x Merrylegs.
  11. (used to indicate choice, as on a ballot, examination, etc.)
  12. (used to indicate an error or incorrect answer, as on a test.)
  13. Mathematics. (in Cartesian coordinates) the x-axis.
  14. Chess. captures.
  15. a person, thing, agency, factor, etc., of unknown identity.



  1. the 24th in order or in a series, or, when I is omitted, the 23rd.
  2. (sometimes lowercase) the Roman numeral for 10.Compare Roman numerals.
  3. Christ.
  4. Christian.
  5. cross.
  6. Electricity. reactance.
  7. Slang. a ten-dollar bill.
  8. (in the U.S.) a rating of the Motion Picture Association of America for movies with subject matter that is suitable for adults only: persons who are under age (usually 17) will not be admitted.Compare G(def 7), PG, PG-13, R1.
  9. (in Great Britain) a designation for a film recommended for adults only.Compare A(def 11), AA(def 5), U(def 6).
  10. a person, thing, agency, factor, etc., of unknown identity.
  11. Chemistry. (formerly) xenon.


noun, plural chis.
  1. the twenty-second letter of the Greek alphabet (X, χ).
  2. the consonant sound represented by this letter.

Origin of chi

From Greek
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for x



noun plural x's, X's or Xs
  1. the 24th letter and 19th consonant of the modern English alphabet
  2. a speech sound sequence represented by this letter, in English pronounced as ks or gz or, in initial position, z, as in xylophone


symbol for
  1. commerce banking finance ex
  2. maths the x- axis or a coordinate measured along the x- axis in a Cartesian coordinate system
  3. an algebraic variable


symbol for
  1. (formerly, in Britain)
    1. indicating a film that may not be publicly shown to anyone under 18. Since 1982 replaced by symbol 18
    2. (as modifier)an X film
  2. denoting any unknown, unspecified, or variable factor, number, person, or thing
  3. (on letters, cards, etc) denoting a kiss
  4. (on ballot papers, etc) indicating choice
  5. (on examination papers, etc) indicating error
  6. for Christ; Christian
  7. (Roman numeral) tenSee Roman numerals

Word Origin for X

(sense 6) from the form of the Greek letter khi (Χ), first letter of Khristos Christ


  1. the 22nd letter of the Greek alphabet (Χ, χ), a consonant, transliterated as ch or rarely kh



ch'i or qi

  1. (sometimes capital) (in Oriental medicine, martial arts, etc) vital energy believed to circulate round the body in currents

Word Origin for chi

Chinese, literally: energy
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for x


most English words beginning in -x- are of Greek origin or modern commercial coinages. East Anglian in 14c. showed a tendency to use -x- for initial sh-, sch- (cf. xal for shall), which didn't catch on but seems an improvement over the current system. As a symbol of a kiss on a letter, etc., it is recorded from 1765. In malt liquor, XX denoted "double quality" and XXX "strongest quality" (1827).

Algebraic meaning "unknown quantity" (1660 in English), sometimes said to be from medieval use, originally a crossed -r-, probably from Latin radix (see root (n.)). Other theories trace it to Arabic, but a more prosaic explanation says Descartes (1637) took x, y, z, the last three letters of the alphabet, for unknowns to correspond to a, b, c, used for known quantities.

Used allusively for "unknown person" from 1797, "something unknown" since 1859. As a type of chromosome, attested from 1902 (first so called in German; Henking, 1891). First used 1950 in Britain to designate "films deemed suitable for adults only;" adopted in U.S. Nov. 1, 1968.



22nd letter of the Greek alphabet, representing a -kh- sound (see ch). The letter is shaped like an X, and so the Greek letter name was used figuratively to signify such a shape or arrangement (e.g. khiasma "two things placed crosswise;" khiastos "arranged diagonally; marked with an X;" khiazein "to mark with an 'X', to write the letter 'X'"). Some dialects used chi to represent the -ks- sound properly belonging to xi; Latin picked this up and the sound value of chi in Latin-derived alphabets is now that of English X.

Online Etymology Dictionary, © 2010 Douglas Harper