X-rays are a high energy type of electromagnetic radiation with wavelengths between 10 picometers and 10 nanometers. The ionizing radiation of an x-ray falls between the range of extreme ultraviolet radiation and gamma rays. X-rays were discovered on Earth by Wilhelm Conrad Röntgen while he was experimenting with high-voltage vacuum tubes. The unit of exposure to x-ray ionizing radiation is called a röntgen in his honor. X-ray photons are used for medical and dental imaging and are produced by a variety of stellar phenomena, including pulsars and quasars.
While working on a way to separate Tuvok and Neelix following their transporter accident, the Doctor referred to the use of barium as a contrast agent for the "primitive imaging technology called x-rays." (VOY: "Tuvix")