What does X stand for in X-Rays?

X-rays were first observed and documented in 1895 by Wilhelm Conrad Röntgen, a German scientist who found them quite by accident when experimenting with vacuum tubes. A week after he first observed them, he took an X-ray photograph of his wife's hand, which clearly revealed her wedding ring and her bones. The photograph electrified the general public and aroused great scientific interest in the new form of radiation. Röntgen called it "X" to indicate it was an unknown type of radiation. The name stuck, although many of his colleagues suggested calling them Röntgen rays. While Röntgen first observed the effects of X-rays in 1895, it wasn't until 1912 that scientists were able to conclude that they were, indeed, another form of light."