X-rays make up X-radiation, a form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 0.1 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 200 keV. X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays.

X-rays with a wavelength greater than 0.1 nm are called soft X-rays. At shorter lengths, they are called hard X-rays. X-rays are distinguished from gamma rays, which are more energetic, depending on their origin: X photons are produced by variations in the kinetics of electrons, while gamma rays by transitions and decays within an atomic nucleus (nuclear origin), or by annihilation between a positron and an electron.