A lot of small words that have the letter X in them do not seem like real words. Xi and xu, for example, are not words that come to mind in everyday usage. Xi is the 14th letter of the Greek alphabet and is also the term identifying the 14th star in a specific constellation. Xu is a Vietnamese coin and equals one-hundredth of a dong.
X is the 24th letter of the English alphabet. It was originally the symbol for samekh, which was a Phoenician letter meaning "fish." The Greeks borrowed the samekh and turned it into their letter "Chi," which was the phonological representation of the digraph, or two letters representing a single sound in speech. Because of this, the letter X, much like C and J, has developed into a letter with myriad pronunciations, depending on its context in a word.
Due to its flexible and undefinable nature, the letter X has become a symbol for the unknown in the terminology of both mathematics and language. One of its most common uses is as the unknown number to solve for in an algebraic equation.Learn more about Education