The Collaborative International Dictionary of English v.0.48:

X \X\ ([e^]ks).
   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, [sect][sect] 217,
   270, 271.
   [1913 Webster]

   Note: The form and value of X are from the Latin X, which is
         from the Greek [Chi], which in some Greek alphabets had
         the value of ks, though in the one now in common use it
         represents an aspirated sound of k.
         [1913 Webster]

WordNet (r) 3.0 (2006):

x
    adj 1: being one more than nine [syn: ten, 10, x]
    n 1: the cardinal number that is the sum of nine and one; the
         base of the decimal system [syn: ten, 10, X,
         tenner, decade]
    2: the 24th letter of the Roman alphabet [syn: X, x, ex]
    3: street names for methylenedioxymethamphetamine [syn: Adam,
       ecstasy, XTC, go, disco biscuit, cristal, X, hug
       drug]

Moby Thesaurus II by Grady Ward, 1.0:

100 Moby Thesaurus words for "X":
   Calvary cross, Christogram, Decalogue, Greek cross,
   Jerusalem cross, John Hancock, Latin cross, Maltese cross,
   Russian cross, T, Ten Commandments, ankh, autograph, avellan cross,
   chi, chi-rho, christcross, cipher, countermark, countersign,
   countersignature, counterstamp, crisscross, cross, cross ancre,
   cross botonee, cross bourdonee, cross fitche, cross fleury,
   cross formee, cross fourchee, cross grignolee, cross moline,
   cross of Cleves, cross of Lorraine, cross patee, cross recercelee,
   cross-crosslet, crossbones, crosslet, crucifix, cruciform, crux,
   crux ansata, crux capitata, crux decussata, crux gammata,
   crux immissa, crux ordinaria, dagger, decade, decagon, decagram,
   decahedron, decaliter, decameter, decare, decastere, decastyle,
   decasyllable, decemvir, decemvirate, decennary, decennium,
   decigram, deciliter, decimeter, decurion, device, endorsement, ex,
   exing, fork cross, gammadion, hand, initials, inverted cross,
   long cross, mark, mark of signature, monogram, papal cross,
   pectoral cross, potent cross, rood, saltire, seal, sigil,
   sign manual, signature, signet, subscription, swastika, tau, ten,
   tenner, trefled cross, visa, vise, voided cross



Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "x":
   dark horse, enigma, frontier, frontiers of knowledge,
   matter of ignorance, mistake, mystery, n, puzzle, riddle,
   sealed book, terra incognita, the incalculable, the strange,
   the unfamiliar, the unknowable, the unknown, unexplored ground,
   unexplored territory, unknown quantity, z



The Jargon File (version 4.4.7, 29 Dec 2003):

X
 /X/, n.

    1. Used in various speech and writing contexts (also in lowercase) in
    roughly its algebraic sense of ?unknown within a set defined by context?
    (compare N). Thus, the abbreviation 680x0 stands for 68000, 68010, 68020,
    68030, or 68040, and 80x86 stands for 80186, 80286, 80386, 80486, 80586 or
    80686 (note that a Unix hacker might write these as 680[0-6]0 and 80[1-6]86
    or 680?0 and 80?86 respectively; see glob).

    2. [after the name of an earlier window system called ?W?] An over-sized,
    over-featured, over-engineered and incredibly over-complicated window
    system developed at MIT and widely used on Unix systems.


The Free On-line Dictionary of Computing (30 December 2018):

X

   1.  Used in various speech and writing contexts
   (also in lowercase) in roughly its algebraic sense of "unknown
   within a set defined by context" (compare N).  Thus, the
   abbreviation 680x0 stands for 68000, 68010, 68020, 68030 or
   68040, and 80x86 stands for Intel 80186, Intel 80286,
   Intel 80386 or Intel 80486.  A Unix hacker might write
   these as 680[0-4]0 and 80[1-4]86 or 680?0 and 80?86
   respectively; see glob.

   2.  An alternative name for the X Window System.

   3.  A suffix for the speed of a CD-ROM drive
   relative to standard music CDs (1x).  32x is common in
   September 1999.

   [Jargon File]

   (1999-09-15)


The Devil's Dictionary (1881-1906):

X in our alphabet being a needless letter has an added invincibility
to the attacks of the spelling reformers, and like them, will
doubtless last as long as the language.  X is the sacred symbol of ten
dollars, and in such words as Xmas, Xn, etc., stands for Christ, not,
as is popular supposed, because it represents a cross, but because the
corresponding letter in the Greek alphabet is the initial of his name
-- _Xristos_.  If it represented a cross it would stand for St.
Andrew, who "testified" upon one of that shape.  In the algebra of
psychology x stands for Woman's mind.  Words beginning with X are
Grecian and will not be defined in this standard English dictionary.