X-ray (n.)

An X-ray is radiation used to see inside things. Wilhelm Röntgen found it in November 1895, working with a covered cathode-ray tube, and announced it in a short paper called On a New Kind of Rays.¹²

He called them X-rays. The X is the one from algebra, the letter you put where a quantity is not known yet. He did not know what the rays were and said so in the name.¹²

German and several other languages call them Röntgen rays instead. He preferred his own placeholder, and English kept it.¹²

The rays were explained long ago. The word still says nobody knows.¹

The X-ray showed a hairline fracture.