X The Unknown

X is typically used as the variable (the unknown quantity) in algebraic equations. According to this post by Dr. Ali Khounsary of the Argonne National Laboratory in Argonne, Illinois, one possible reason why the letter X is used to denote the unknown factor in algebraic equations dates back to the origins of algebra itself in Arabic civilization. Dr. Khounsary writes:

Algebra has its roots in the Middle East where sciences including mathematics and astronomy flourished in the Islamic world in the 700-1450 period. Muhammad al-Khwarizmi (780­-850) was one of the major mathematicians of his time and the author of a number of influential books. One of his major books is on arithmetic and another on algebra. In fact, it is his transmuted name ‘algorithm’ which we now use to refer to the step-by-step procedures for solving a problem. His algebra book is titled Kitab al-jabr wal-muqabala which translates to “the book of calculation by completion and reduction.” The Arabic word “al-jabr” is the origin of the word “algebra” which describes the process of moving terms from one side of an algebraic equation to the other to find the value of an unknown. . . .

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.

Xenos, of course, is the root of the word xenophobia: the fear of foreigners or strangers. Interestingly, as it happened, the call letters of all Mexican radio stations—also referred to as “border radio”—begin with an X. The first Mexican radio station, located in Reynosa, started broadcasting in 1930, with the call letters XED, possibly a pun on “crossed [Xed] out,” a reference to the marginalized and dispossessed. Only one major company uses X in its name—Xerox—and very few bands have used X in the group’s proper name—X-ray Spex (a play on "X-ray Specs," ads for which used to appear in comic books, pictured), the L. A. punk band X, and XTC come to mind—and very few albums and songs have used X in the title. One should perhaps remember that X is also an abbreviation for Christ, as in “Xmas,” and that once John Lennon named an album Shaved Fish, perhaps a play on ICTHUS, the Greek word for “fish”—ICTHUS being an acrostic referring to Jesus Christ, the Son of God.

Some Albums:
Electric Light Orchestra, Xanadu
David Lindley, El Rayo-X
Iron Maiden, The X Factor
Olivia Newton-John, Xanadu
Def Leppard, X
Mushroomhead, XX
Toto, XX (1977-97)
ZZ Top, XXX

Some Songs:
Blondie, “X Offender”
Coldplay, “X & Y”
John Lennon, “Happy Xmas (War Is Over)”
Mushroomhead, “Xeroxed”
Olivia Newton-John, “Xanadu”
Rush, “Xanadu”
System Of A Down, “X”
U2, “Xanax and Wine”
Frank Zappa, “Project X”
ZZ Top, “Heard It On The X”