X is the twenty-fourth letter of the Latin alphabet. It is in the form of St Andrew's Cross. In the NATO phonetic alphabet it is X-Ray.
/ks/ was in Ancient Greece written as Chi 'Χ' (Western Greek) or Xi 'Ξ' (Eastern Greek). In the end, Chi was standardized as /k_h/ (/x/ in Modern Greek) as well as Xi for /ks/. But the Etruscans took over X from older Western Greek, therefore, it stood for /ks/ in Etruscan and /ks/ and /gs/ [gz] in Latin. Only in Azeri, Uzbek and Tatar is X pronounced [kh]. Some scholars claim that Latin X is not identical with Greek X.
It is also controversial whether Psi, Chi (Khi) and Xi are Greek inventions or whether they are ultimately of Semitic origin.
X-ray represents the letter X in the NATO phonetic alphabet.
In context, X is also used as or in:
- The Roman numeral for ten.
- In elementary algebra, x commonly represents an unknown variable. Even though any letter can be used, x is the most common by far. This usage can be traced back to the Arabic word ?ay 'thing' which was spelled with an initial x in Old Spanish or (according to other sources) an abbreviation of Latin causa which was the translation of Arabic š.
- In a general sense, to represent the unknown or secret, as in project X and in the context of X rays, a form of electromagnetic radiation. Planet X is a hypothetical additional planet farther from the sun.
- Reference to sex, such as the X factor (this could of course refer to the unknown factor).
- Abbreviation for extra, such as XL for extra large size
- Abbreviation for Christ, as in Xmas (Christmas) and xtian (Christian).
- The stock symbol for United States Steel Corporation.
- Other abbreviations, such as Xtal (Crystal), reXn (reaction) and Xperimental (experimental).
- A represention for a cross in games like tic tac toe and in words like Xing (crossing), or to sign with a cross (often for illiteracy). Crossing something out with an X denotes elimination, prohibition or erasure.
- X Window System, a free graphical windowing system developed at MIT
- Slang for the drug MDMA, commonly called Ecstasy
- The X chromosome.
- Generation X
- X-rated, a US system for rating films for age groups. Also X was the old certificate in the UK for 18.
- Exact time in photographic flash synchronization
- A punk band (see X (band))
- X, a German film made in 1928
- X-Men, the superheros
- The X-files
- Several radio stations, such as 99X in Atlanta and 91X in Tijuana/San Diego
- The variable for reactance
- A kiss
- The pseudonym of the person who blew the whistle on the Watergate crisis
- A nickname for the École polytechnique
- A symbol for an alcoholic proof of 50; multiple Xes indicate multiples of 50
- The US Navy hull classification symbol for Submersible Craft