The 'X' in X-rays stands for unknown. It is a long story when these rays were discovered by the German physicist, Wilhelm Conrad Röntgen in 1895. He named them X-rays because he didn’t know their nature what it was.
The nature of these radiations was unknown to him and like in algebra an unknown quantity is represented by the letter ‘X’. So, he called them X-rays. These rays had the ability to pass through many materials that absorb visible light.
X-rays also have the potential to surpass loose electrons from atoms. In the present time, these rays have exceptional properties that has made X-rays useful in many fields, such as medicine and research into the nature of the atom.