X

X is the twenty-fourth letter of the Latin alphabet.

A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z

/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. 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.

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.
• Abbreviation for extra, such as XL for extra large size
• Abbreviation for Christ, as in Xmas (Christmas) and xtian (Christian).
• 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).
• X Window System, a free graphical windowing system developed at MIT
• Slang for the drug MDMA, commonly called Ecstasy
• X rays, a form of electromagnetic radiation
• The X chromosome.
• Generation X
• X-rated, a US system for rating films for age groups.
• Exact time in photographic flash synchronization
• A punk band (see X (band))
• X[?], a German film made in 1928
• X-Men, the superheros