X-ray

Concise Oxford English Dictionary © 2008 Oxford University Press:
X-ray/ˈeksreɪ/

▶noun

• 1 an electromagnetic wave of very short wavelength (between ultraviolet and gamma rays), able to pass through many materials opaque to light.

  ■ [as modifier] informal denoting an apparent or supposed faculty for seeing beyond an outward form: X-ray eyes.

• 2 a photograph or other image of the internal structure of an object, especially a part of the body, produced by passing X-rays through the object.

• 3 a code word representing the letter X, used in radio communication.

▶verb photograph or examine with X-rays.

– origin translation of Ger. X-Strahlen (pl.), from X- (because, when discovered in 1895, the nature of the rays was unknown) + Strahl ‘ray’.