x

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The 24th letter of the modern English alphabet.
• noun Any of the speech sounds represented by the letter x.
• noun The 24th in a series.
• noun Something shaped like the letter X.
• noun A mark inscribed to represent the signature of one who is unable to sign one's name.
• noun An unknown or unnamed factor, thing, or person.
• transitive verb To mark or sign with an X.
• transitive verb To delete, cancel, or obliterate with a series of X's. Often used with out.

from The Century Dictionary.

• The chemical symbol of xenon.
• In electricity, the symbol for reactance in alternating-current circuits.
• In wireless telegraphy, a local disturbance causing a false signal; a stray.
• The twenty-fourth letter and nineteenth consonant-sign in the English alphabet.
• As a numeral, X stands for ten.
• As an abbreviation, X. stands for Christ, as in Xn. (Christian), Xmas. (Christmas).
• As a symbol: In ornithology, in myological formulas, the symbol of the semitendinosus muscle.
• In mathematics:
• [lowercase] In algebra, the first of the unknown quantities or variables.
• [l. c] In analytical geometry, an abscissa or other rectilinear point-coördinate.
• In mechanics, the component of a force in the direction of the axis of x.
• Originally, a mark on brewers' casks; hence, a name given to ale of a certain quality. Compare XX, XXX.

from the GNU version of the Collaborative International Dictionary of English.

• X, the twenty-fourth letter of the English alphabet, has three sounds; a compound nonvocal sound (that of ks), as in wax; a compound vocal sound (that of gz), as in example; and, at the beginning of a word, a simple vocal sound (that of z), as in xanthic. See Guide to Pronunciation, §§ 217, 270, 271.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The twenty-fourth letter of the basic modern Latin alphabet.
• noun voiceless velar fricative.
• noun mathematics An unknown variable.
• noun mathematics The first coordinate in Cartesian coordinates, representing horizontal position.
• noun A kiss at the end of a letter, or similar missive.
• noun The twenty-fourth letter of the English alphabet, called ex and written in the Latin script.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the cardinal number that is the sum of nine and one; the base of the decimal system
• noun street names for methylenedioxymethamphetamine
• adjective being one more than nine
• noun the 24th letter of the Roman alphabet