In ancient times, some dialects of Greek used the chi instead of xi to represent the /ks/ sound. This was borrowed into the early Latin language, which led to the letter X being used for the same sound in Latin, and the modern languages which use the Latin alphabet.
Chi also was included into Cyrillic alphabet (as letter Х), with the phonetic value /x/ or /h/.
In Plato's Timaeus, it is explained that the two bands which form the soul of the world cross each other like the letter Χ.
Chi or X is often used to abbreviate the name Christ, as in the holiday Christmas (Xmas).
The optic chiasm, an x-shaped connection of the optic nerves leading to the eye, got its name from the letter chi because of its shape; [1] likewise, the shape of the letter chi is the origin of the rhetorical devicechiasmus.
References
^Asimov, Isaac (1963). The Human Brain. Boston: Houghton Mifflin.