1. The 24th. letter of the English alphabet often used to indicate something that is not known or not learned: Alisa wrote a large X instead of a formal signature because she didn't know how to write her name.
2. Etymology: the use of the letter x to indicate "unknown quantity" goes back to the French philosopher-mathematician Rene Descartes, who introduced x, y, z, and such usage into the mathematical discourse proceeding from an originally Arabic paradigm (standard or typical example).

By the 18th century, the usage was extended from math to everyday language, and X in the sense of "unknown person" has been in our language ever since.

The term x-rays was used by its discoverer, Wilhelm Roentgen, because their nature and sources were unknown.

A short term meaning "extra small": An XS is a symbol that is presented to specify clothing sizes.