X射線（又被稱為艾克斯射線、倫琴射線或X光）是一種波長範圍在0.1奈米到10奈米之間（對應頻率範圍30 PHz到3EHz））的電磁輻射形式。X射線最初用於醫學成像診斷和 X射線結晶學。X射線也是游離輻射 等這一類對人體有危害的射線。
範例(英)：X-radiation (composed of X-rays) is a form of electromagnetic radiation. X-rays have a wavelength in the range of 10 to 0.01 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3 × 1016 Hz to 3 × 1019 Hz) and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays. In many languages, X-radiation is called Röntgen radiation after Wilhelm Conrad Röntgen, who is generally credited as their discoverer, and who had called them X-rays to signify an unknown type of radiation.
