Source : Webster's Revised Unabridged Dictionary (1913)

Rontgen ray \R["o]ntgen ray\ (Physics)
   Any of the rays produced when cathode rays strike upon
   surface of a solid (as the wall of the vacuum tube).
   R["o]ntgen rays are noted for their penetration of many
   opaque substances, as wood and flesh, their action on
   photographic plates, and their fluorescent effects. They were
   called {X rays} by their discoverer, W. K. R["o]ntgen. They
   also ionize gases, but cannot be reflected, or polarized, or
   deflected by a magnetic field. They are regarded as
   nonperiodic, transverse pulses in the ether. They are used in
   examining opaque objects, as for locating fractures or
   bullets in the human body.

X rays \X rays\, or X-rays \X"-rays`\, n. pl.
   The R["o]ntgen rays; -- so called by their discoverer because
   of their enigmatical character.