The "X" in X-rays is not an abbreviation. It was chosen by the German physicist Wilhelm Conrad Röntgen when he discovered these rays in 1895.
He called them X-rays because the letter "X" is commonly used in mathematics and science to represent an unknown quantity. At the time of the discovery, he did not know what kind of radiation it was, so he gave it the temporary name X-rays, meaning "unknown rays."
Later, scientists discovered that X-rays are a form of high-energy electromagnetic radiation. They can pass through soft tissues but are absorbed more by bones, which is why they are used to produce medical images such as radiographs.
In many European countries, especially Germany, X-rays are also called Röntgen rays or simply Röntgen, in honor of Wilhelm Conrad Röntgen.