X-rays are a form of electromagnetic radiation with a wavelength in the range of 10 to 0.1 nanometers, corresponding to frequencies in the range 30 to 3000 PHz. X-rays are primarily used for diagnostic medical imaging and crystallography. X-rays are a form of ionizing radiation and as such can be dangerous.

In many languages it is called Röntgen radiation after its discoverer Wilhelm Röntgen.