TRIVIAL PURSUITS: Why are X-rays called X-rays?

Why are the rays that send these particles called “X-rays?”

The X-ray was discovered in 1895 by a German Professor of physics at the University of Wurzburg in Germany, Wilhelm Konrad Roentgen. He did not understand the nature of these rays.

In mathematics, “x” represents an unknown quantity. Roentgen called the rays X-rays, representing that which had not yet been explained by science.