The "x" in x-ray was used because the scientist who discovered x-rays, Wilhelm Conrad Rontgen, didn't know the nature of the rays; like in a mathematical equation, they represented an unknown. The x-ray was discovered in 1895.
Physicist Wilhelm Conrad Rontgen made the discovery of the x-ray completely by accident. His original experiment was to test whether cathode rays could pass through glass. He noticed a glow coming from a screen that had been treated with a variety of chemicals but had no idea how the glow appeared.
What Rontgen didn't know was that x-rays are electromagnetic and act a lot like light rays. After his accidental discovery, Rontgen spent time in his lab conducting numerous experiments, trying to find out the nature of these mysterious rays. It wasn't long before he figured out that x-rays could see through human flesh, and x-rays quickly began being used to find bullets and broken bones during the Balkan War. In 1901, Rontgen received the Nobel Prize in physics for his discovery. It wasn't until much later that scientists realized the harmful effects of the radiation used to create x-rays.
Today, x-rays are used in completely safe ways and have proven to be useful in the medical field and in other areas such as airport security and material analysis.