- a band of electromagnetic radiation with wavelengths between gamma rays and ultraviolet radiation (c. .005 to c. 5 nanometers)
- a stream of electromagnetic waves within this band: X-rays are capable of penetrating opaque or solid substances, ionizing gases, and, by extended exposure, destroying organic tissue: they are widely used in medicine for diagnosis and treatment of certain organic disorders
- a photograph made by means of X-rays
Origin of X-ray
translated, translation of German x-strahl ( from x, algebraic symbol for an unknown quantity + strahl, ray): so named by W. C. Roentgen (1895), because of its unknown character