x 字密在內,

In algebraic equations, one solves equations to obtain the value(s) of one or more unknown(s).  The word for "thing" or "object" (presumably unknown thing or object) in Arabic - which was the principal language of sciences during the Islamic civilization - is "shei" which was translated into Green as xei, and shortened to x, and is considered by some to be the reason for using x. It is also noteworthy that "xenos" is the Greek word for unknown, stranger, guest, or foreigner, and that might explain the reasons  Europeans used the letter x to denote the "unknown” in algebraic equations.

Dr. Ali Khounsary    Advanced Photon Source   Argonne National Laboratory   Argonne, IL