X-radiation (or X-rays) is the penetrating electromagnetic radiation (photons) with a wavelength from 0.01 to 1.0 nanometres that is much shorter than visible light, but longer than gamma rays.

Reference Definition by Wikipedia: X-radiation (composed of X-rays) is a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz) and energies in the range 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. In many languages, X-radiation is referred to with terms meaning Röntgen radiation, after the German/Dutch scientist Wilhelm Röntgen, who usually is credited as its discoverer, and who had named it X-radiation to signify an unknown type of radiation.

Related Definitions in the Project: The Science Definitions; Property and Unit