X is the twenty-fourth letter of the Latin alphabet. It is also the form of St Andrew's Cross .
/ks/ was in Ancient Greece written as Chi 'Χ' (Western Greek) or Xi 'Ξ' (Eastern Greek). In the end, Chi was standardized as (the voiceless velar fricative /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 Kurdish, Azeri, Uzbek, Tatar and Lojban as well as the IPA and SAMPA (the latter is used here) is X pronounced [x] (as in German Bach). Some scholars claim that Latin X is not identical with Greek Χ.
It is also controversial whether Psi, Chi (Khi) and Xi are Greek inventions or whether they are ultimately of Semitic origin.
The letter X is not used in the Italian language, except for words borrowed from other languages, or names of foreigners. No words in the Basic English vocabulary begin with X, but it occurs in words beginning with other letters.
In philosophy, X may mean the supreme singular guiding principle, the logos, for some people. Especially for skeptics, some of whom have a principle which they don't want to name or are unsure of, or want to be neutral and not take any side, they use the general letter X to denote their principle. There is also a formal movement formed urging people to embrace the X principle, form a united world government based upon it, and strive for longeivity and material immortality, drawing upon the thoughts of the Bangladeshi writer Md. Shahruz Zahrat and some others.
In a general sense, X represents an unknown or secret, as in project X or mister X. This usage is borowed from mathematics (see below) and acquired some specialized meanings, listed here.
Members of the Nation of Islam change their surnames to "X" to symbolize that their African names were lost in slavery, an example is Malcolm X.
In aeronautics, X is the designation given to an experimental aircraft of the US government, for instance, the X-1 rocketplane that first broke the sound barrier. It is also a prefix to experimental types of US military aircraft, for instance, the XB-70.
In anime, X is a series produced by CLAMP based on the Japanese mangaX/1999 (see X (anime)) which has also been adapted into a 1996 feature film and a 2001 television series.
X is often used as a symbol of multiplication, as in denoting the spin (and, later, transfer) speed of CDs and DVDs compared to an original standard speed. This usage is a corruption of the multiplication symbol ×. The data transfer speed for 1X CDs is 153,600 bytes/second and one for 1X DVDs is 1,385,000 bytes/second (approximately 9.02 times faster).
X is used as an abbreviation for the Apple operating system Mac OS X.
X is a rating given to films suitable for an adult-only audience; see X-rated. NC-17 has replaced the X rating in the US. The UK replaced the X rating with the 18 certificate. Australia retains the X rating.
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.
x is the usual symbol for the variable represented on the horizontal axis (ordinate) in analytic geometry.
X is a symbol worn on the hand to denote that someone is straight edge. It is also frequently tattooed on other parts of the body, or worn on clothing, sometimes in triplicate (XXX). Straight-edgers frequently also append Xs to their names, i.e. write their names with Xs, such that 'Jack' would be written XjackX. Straight edge itself is comonly abbreviated to 'sxe' (S.E. with an X in the middle).
