X-ray
n. an electromagnetic emission of short wavelength produced by bombarding a heavy metal target, such as tungsten, with high-energy electrons in a vacuum tube. X-rays are used for diagnostic purposes to visualize internal body structures: The radiation can penetrate most substances and produce images of objects on photographic film (see radiography) or can cause certain chemicals to fluoresce. Prolonged or unnecessary exposure can be extremely damaging; therefore, when X-rays are used therapeutically for diagnosis or to destroy malignant cells (see radiation therapy), great precautions are taken to limit and target exposure. Also called roentgen ray.