In algebra, the letter 'x' is often used to represent an unknown quantity or variable. Similarly, in English, x represents the unknown, as in X-rays, which baffled their discoverer, and Malcolm X, who chose the symbol to represent the forgotten name of his African ancestors.
This meaning of the letter x traces back to the Arabic word for "thing," or šay'. In ancient texts, such as Al-Jabr, a manuscript written in Baghdad in 820 A.D. that established the rules of algebra, mathematical variables were called things. (An equation might read "three things equal 15," for example — the thing being five.)
When Al-Jabr was later translated into Old Spanish, the word šay' was written as "xei." This soon came to be abbreviated as x.