From 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]

From WordNet (r) 2.0:

x adj : being one more than nine [syn: ten, 10] 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]

From 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

From 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

From Jargon File (4.3.1, 29 Jun 2001):

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.

From The Free On-line Dictionary of Computing (27 SEP 03):

X 1. <convention> 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. <graphics> An alternative name for the X Window System. 3. <storage> 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)