Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

Source : WordNet®

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

Source : Free On-Line Dictionary of Computing

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)