X
X, or x, is the 24th and third-to-last letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its  name in English is "ex" (pronounced ), plural exes.

History
In Ancient Greek, 'Χ' and 'Ψ' were among several variants of the same letter, used originally for /kʰ/ and later, in western areas such as Arcadia, as a simplification of the digraph 'ΧΣ' for /ks/. In the end, more conservative eastern forms became the standard of Classical Greek, and thus 'Χ' (Chi) stood for /kʰ/ (later /x/; palatalized to [ç] in Modern Greek before front vowels). However, the Etruscans had taken over 'Χ' from western Greek, and it therefore stands for /ks/ in Etruscan and Latin.
The letter 'Χ' ~ 'Ψ' for /kʰ/ was a Greek addition to the alphabet, placed after the Semitic letters along with phi 'Φ' for /pʰ/.

Pronunciation and use
English
In English orthography, ⟨x⟩ is typically pronounced as the voiceless consonant cluster  when it follows the stressed vowel (e.g. ox), and the voiced consonant  when it precedes the stressed vowel (e.g. exam). It is also pronounced  when it precedes a silent ⟨h⟩ and a stressed vowel (e.g. exhaust). Before ⟨a⟩, ⟨i⟩ or ⟨u⟩, it can be pronounced  or  (e.g. sexual and luxury); these result from earlier  and . It also makes the sound  in words ending in -xion (except for axion). When ⟨x⟩ ends a word, it is always  (e.g. fax), except in loan words such as faux (see French, below).
There are very few English words that start with ⟨x⟩ (the fewest of any letter). When ⟨x⟩ does start a word, it is usually pronounced 'z' (e.g. xylophone, xenophobia, and xanthan). When starting in some names or as its own representation it is pronounced 'eks', in rare recent loanwords or foreign proper names, it can also be pronounced  (e.g. the obsolete Vietnamese monetary unit xu) or  (e.g. Chinese names starting with Xi like Xiaomi or Xinjiang). Many of the words that start with ⟨x⟩ are of Greek origin, or standardized trademarks (Xerox) or acronyms (XC). In abbreviations, it can represent "trans-" (e.g. XMIT for transmit, XFER for transfer), "cross-" (e.g. X-ing for crossing, XREF for cross-reference), "Christ-"  (e.g. Xmas for Christmas, Xian for Christian), the "crys-" in crystal (XTAL), "by" (SXSW for South by Southwest) or various words starting with "ex-" (e.g. XL for extra large, XOR for exclusive-or, or the extinction symbol).
X is the third least frequently used letter in English (after ⟨q⟩ and ⟨z⟩), with a frequency of about 0.15% in words.

Other languages
In Latin, ⟨x⟩ stood for [ks]. In some languages, as a result of assorted phonetic changes, handwriting adaptations or simply spelling convention, ⟨x⟩ has other pronunciations:

In Basque, ⟨x⟩ represents [ʃ]. Additionally there is the digraph ⟨tx⟩ [tʃ].
In Dutch, ⟨x⟩ usually represents [ks], except in the name of the island of Texel, which is pronounced Tessel. This is because of historical sound-changes in Dutch, where all /ks/ sounds have been replaced by /s/ sounds. Words with an ⟨x⟩ in the Dutch language are nowadays usually loanwords. In the Dutch-speaking part of Belgium, family names with ⟨x⟩ are not uncommon (e.g. Dierckx, Hendrickx, Koninckx, Sterckx, Vranckx).
In Norwegian, ⟨x⟩ is generally pronounced [ks], but since the 19th century, there has been a tendency to spell it out as ⟨ks⟩; it may still be retained in personal names, though it is fairly rare, and occurs mostly in foreign words and SMS language. Usage in Danish and Finnish is similar (while Swedish, on the other hand, makes frequent use of ⟨x⟩ in native words as well as in loanwords).
In German, generally pronounced [ks]; in native words, however, such as Ochs or wachsen,  the cluster [ks] is often written ⟨chs⟩.
French: at the ends of words, silent (or [z] in liaison if the next word starts with a vowel). Three exceptions are pronounced [s]: six ("six"), dix ("ten") and in some city names such as Bruxelles (although some people pronounce it 'ks') or Auxerre; it is fully pronounced [ks] in Aix, the name of several towns. It is pronounced [z] in sixième and dixième. Otherwise [ks] or (primarily in words beginning with ex- followed by a vowel) [ɡz].
In Italian, ⟨x⟩ is either pronounced [ks], as in extra, uxorio, xilofono, or [ɡz], as exogamia, when it is preceded by ⟨e⟩ and followed by a vowel. In several related languages, notably Venetian, it represents the voiced sibilant [z]. It is also used, mainly amongst the young people, as a short written form for "per", meaning "for": for example, "x sempre" ("forever"). This is because in Italian the multiplication sign (similar to ⟨x⟩) is called "per". However, ⟨x⟩ is found only in loanwords, as it is not part of the standard Italian alphabet; in most words with ⟨x⟩, this letter may be replaced with 's' or 'ss' (with different pronunciation: xilofono/silofono, taxi/tassì) or, rarely, by 'cs' (with the same pronunciation: claxon/clacson).
In Old Spanish, ⟨x⟩ was pronounced [ʃ], as it is still currently in other Iberian Romance languages. Later, the sound evolved to a hard [x] sound. In modern Spanish, due to a spelling reform, whenever ⟨x⟩ is used for the [x] sound it has been replaced with ⟨j⟩, including in words that originally had ⟨x⟩ such as ejemplo or ejercicio, though ⟨x⟩ is still retained for some names (notably 'México', even though 'Méjico' may sometimes be used in Spain). Presently, ⟨x⟩ represents the sound [s] (word-initially), or the consonant cluster [ks] (e.g. oxígeno, examen). Rarely, it can be pronounced [ʃ] as in Old Spanish in some proper nouns such as 'Raxel' (a variant of Rachel) and Uxmal.
In Galician and Leonese, ⟨x⟩ is pronounced [ʃ] in most cases (often used in place of etymological g or j). The pronunciation [ks] occurs in learned words, such as 'taxativo' (taxing). However, Galician speakers tend to pronounce it [s], especially when it appears before plosives, such as in 'externo' (external).
In Catalan, ⟨x⟩ has three sounds; the most common is [ʃ]; as in 'xarop' (syrup). Other sounds are: [ks]; 'fixar' (to fix), [ɡz]; 'examen'. In addition, [ʃ] gets voiced to [ʒ] before voiced consonants; 'caixmir'. Catalan also has the digraph ⟨tx⟩, pronounced [tʃ].
In Portuguese, ⟨x⟩ has four main sounds; the most common is [ʃ], as in 'xícara' (cup). The other sounds are: [ks] as in 'flexão' (flexion); [s], when preceded by E and followed by a consonant, as in 'contexto' ([ʃ] in European Portuguese), and in a small number of other words, such as 'próximo' (close/next); and (the rarest) [z], which occurs in the prefix 'ex-' before a vowel, as in 'exagerado' (exaggerated). A rare fifth sound is [ɡz], coexisting with [z] and [ks] as acceptable pronunciations in exantema and in words with the Greek prefix 'hexa-'.
In Sardinian and Ligurian, X represents [ʒ].
In Venetian, it represents the voiced alveolar sibilant [z] much like in Portuguese 'exagerado', English 'xylophone' or in the French 'sixième'. Examples from medieval texts include raxon (reason), prexon (prison), dexerto (desert), chaxa or caxa (home). Nowadays, the best-known word is xe (is/are). The most notable exception to this rule is the name Venexia [veˈnɛsja] in which ⟨x⟩ has evolved from the initial voiced sibilant [z] to the present day voiceless sibilant.
In Albanian, ⟨x⟩ represents [dz], while the digraph ⟨xh⟩ represents [dʒ].
In Maltese, ⟨x⟩ is pronounced [ʃ] or, in some cases, [ʒ] (only in loanwords such as 'televixin', and not for all speakers).
In Polish, ⟨x⟩ was used prior to 19th century both in loanwords and native words and was pronounced [ks] or [ɡz], e.g. xiążę, xięstwo (now książę, księstwo). Later was replaced by ⟨ks⟩ and ⟨gz⟩ in almost all words and remained only in a few loanwords as 'xenia' (xenien), surnames as Axentowicz, Jaxa, Koxowski, Mixtacki, Rexemowski, Xiężopolski, names as Xawery, Xymena and abbreviations.Additionally, in languages for which the Latin alphabet has been adapted only recently, ⟨x⟩ has been used for various sounds, in some cases inspired by European usage, but in others, for consonants uncommon in Europe. For these no Latin letter stands out as an obvious choice, and since most of the various European pronunciations of ⟨x⟩ can be written by other means, the letter becomes available for more unusual sounds.

⟨x⟩ represents [x] (voiceless velar fricative) in e.g. Azerbaijani, Kurdish (Hawar alphabet), Georgian (when Latinized), Lojban, Pashto (when Latinized), Tatar (Jaꞑalif, Zamanälif, official romanization of 2012), Uzbek, and Uyghur (Latin script).
Esperanto: The x-convention replaces ⟨ĉ⟩, ⟨ĝ⟩, ⟨ĥ⟩, ⟨ĵ⟩, ⟨ŝ⟩, and ⟨ŭ⟩ with x-suffixes: ⟨cx⟩, ⟨gx⟩, ⟨hx⟩, ⟨jx⟩, ⟨sx⟩, and ⟨ux⟩.
In transliteration of Indian languages, primarily Indo-Aryan languages, ⟨x⟩ represents the consonant cluster [kʃ] in alternate spellings of words containing 'क्ष' (kṣ), especially names such as Laxmi and Dixit. Less frequently, ⟨x⟩ is used to represent 'ख़' [x].
In Apache ⟨x⟩ represents [x]
In Nahuatl, ⟨x⟩ represents [ʃ].
In Nguni languages, ⟨x⟩ represents the alveolar lateral click [ǁ].
In Pirahã, ⟨x⟩ symbolizes the glottal stop [ʔ].
An illustrating example of "x" as a "leftover" letter is differing usage in three different Cushitic languages:
Afar: voiced alveolar implosive [ɗ]
Oromo: alveolar ejective [tʼ]
Somali: voiceless pharyngeal fricative [ħ]
In East and Southeast Asia:
In Lao, based on romanization of Lao consonants, ⟨x⟩ may represent [ɕ], e.g. in Lan Xang.
In Vietnamese, ⟨x⟩ is pronounced like English ⟨s⟩ (at the beginning of a word, e.g. "sing"). This sound was [ɕ] in Middle Vietnamese, resembling the Portuguese sound /ʃ/, spelled ⟨x⟩.
In Hanyu Pinyin, Standard Chinese's official transcription system in China, Malaysia, Singapore, and Taiwan, the letter ⟨x⟩ represents the voiceless alveolo-palatal fricative /ɕ/, for instance in 'Xi', [ɕi].

Other systems
In the International Phonetic Alphabet, ⟨x⟩ represents a voiceless velar fricative.

Other uses
In mathematics, x is commonly used as the name for an independent variable or unknown value. The modern tradition of using x, y and z to represent an unknown (incognita) was introduced by René Descartes in La Géométrie (1637). As a result of its use in algebra, X is often used to represent unknowns in other circumstances (e.g. X-rays, Generation X, The X-Files, and The Man from Planet X; see also Malcolm X).
On some identification documents, the letter X represents a non-binary gender, where F means female and M means male.In the Cartesian coordinate system, x is used to refer to the horizontal axis.
It is also sometimes used as a typographic approximation for the multiplication sign, ×. In mathematical typesetting, x meaning an algebraic variable is normally in italic type (
  
    
      
        x
        
      
    
    {\displaystyle x\!}
  ), partly to avoid confusion with the multiplication symbol. In fonts containing both x (the letter) and × (the multiplication sign), the two glyphs are dissimilar.
It can be used as an abbreviation for 'between' in the context of historical dating; e.g., '1483 x 1485'.
Maps and other images sometimes use an X to label a specific location, leading to the expression "X marks the spot".The Roman numeral X represents the number 10.The Suzhou numeral 〤 represents the number 4.
In art or fashion, the use of X indicates a collaboration by two or more artists, e.g. Aaron Koblin x Takashi Kawashima. This application, which originated in Japan, now extends to other kinds of collaboration outside the art world. This usage mimics the use of a similar mark in denoting botanical hybrids, for which scientifically the multiplication × is used, but informally a lowercase "x" is also used.
At the end of a letter or other correspondence, 'x' can mean a kiss; the earliest example of this usage cited by the Oxford English Dictionary is from 1878.An X rating denotes media such as movies that are intended for adults only.

Related characters
Descendants and related characters in the Latin alphabet
X with diacritics: Ẍ ẍ Ẋ ẋ X̂ x̂ ᶍ
IPA-specific symbols related to X: χ
Teuthonista phonetic transcription-specific symbols related to X:U+AB56 ꭖ LATIN SMALL LETTER X WITH LOW RIGHT RING
U+AB57 ꭗ LATIN SMALL LETTER X WITH LONG LEFT LEG
U+AB58 ꭘ LATIN SMALL LETTER X WITH LONG LEFT LEG AND LOW RIGHT RING
U+AB59 ꭙ LATIN SMALL LETTER X WITH LONG LEFT LEG WITH SERIF
ˣ : Modifier letter small x is used for phonetic transcription
ₓ : Subscript small x is used in Indo-European studies

Ancestors and siblings in other alphabets
Χ χ : Greek letter Chi, from which the following derive:
Ꭓ ꭓ : Latin chi
Х х : Cyrillic letter Kha
Ⲭ ⲭ : Coptic letter Khe, which derives from Greek Chi
𐍇 : Gothic letter enguz, which derives from Greek Chi
𐌗 : Old Italic X, which derives from Greek Chi, and is the ancestor of modern Latin X
ᚷ : Runic letter Gyfu, which may derive from old Italic X
Ξ ξ : Greek letter Xi, which was used in place of Chi in the Eastern (and the modern) Greek alphabets

Computing
Computing codes
1 Also for encodings based on ASCII, including the DOS, Windows, ISO-8859 and Macintosh families of encodings.In the C programming language, "x" preceded by zero (as in 0x or 0X) is used to denote hexadecimal literal values.
X is commonly used as a prefix term in nouns related to the X Window System and Unix.

Other representations
See also
X mark

References
External links
 Media related to X at Wikimedia Commons
 The dictionary definition of X at Wiktionary
 The dictionary definition of x at Wiktionary
"X" . The American Cyclopædia. 1879.