X

X is the twenty-fourth (number 24) letter in the English alphabet. It makes the kss sound (for example: exit, next, fox), or sometimes the gzz sound, or sometimes, if it is at the beginning of a word, the zzz sound. The language of origin is from the Greek and Latin language.

• In a general sense, X means something is unknown or secret, as in project X.
• X is also used often to show kisses, usually at the end of a letter or text message.
• In clothing, X is used as a short letter for extra, such as XXL for extra-extra-large.
• In Roman numerals, X means ten.
• In internet terms, XXX indicates adult material, as in an XXX site.
• In x-ray, X stands for its name, which is "ex", even though X is at the end of it.
• X is a common variable used in algebra.
• X, formerly Twitter

The dictionary definition of x at Wiktionary
Media related to X at Wikimedia Commons