X marks the spot today. That's because today is X Ray Discovery Day, commemorating the anniversary of the day in 1895 when German physicist Wilhelm Conrad Roentgen ushered in a new era in physics and medicine with his nearly accidental discovery of the X strahl.

Strahl translates from German as "ray," while the X in X strahl comes from the long-established tradition of using "X" to designate the mathematically unknown. Why "X"? French mathematician and philosopher René Descartes introduced it, together with "Y" and "Z," as symbols for unknowns corresponding to the symbols "A," "B," and "C" of knowns, in his book entitled Geometry.

Although the term X ray has largely supplanted the synonymous roentgen-ray, the name of its discoverer does live on in the measurement roentgen. Since 1922, the international scientific community has been using roentgen for the name of one unit of X radiation or gamma radiation.

But let's get back to the "X." We mentioned our mathematical "X" owes a debt to a French philosopher; we can thank the Greeks for the religious "X" that symbolizes Christ and Christian, and which turns up in the sometimes maligned term Xmas. That "X" is a transliteration of the Greek chi, the first letter of Christ.