Electromagnetic radiation of high energy which results from either the release of energy from electrons changing orbits about the nucleus (discrete) or the inelastic collision of charged particles with the electromagnetic field of the nucleus. X-rays have wavelengths shorter than those in the ultraviolet region, e.g., less than 10E-6 cm or 100 Angstroms. Materials at very high temperatures (millions of degrees) emit such radiations; they are then called thermal x-rays. As generally produced by x-ray machines, they are "bremsstrahlung" resulting from the interaction of electron of 1 kiloelectron-volt or more energy with a metallic target. (See Electromagnetic Radiation and Thermal X-Rays.)