1. a letter of the alphabet
/ks/ was in Ancient Greece written as X >CHI> or KSI> . In the end, X 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.
2. In mathematics, 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.
4. The X Window System is free software developed by MIT which allows various programs to display windows on graphical terminals. It is most commonly used on Unix and derivatives but is available for most other operating systems as well.
5. X represents the number 10 (ten) in the Roman numerals.