The Collaborative International Dictionary of English v.0.48:

Rontgen ray \R["o]ntgen ray\, Roentgen ray \Roentgen ray\ [see
   R["o]ntgen.] (Physics)
   An X-ray; originally, the term was applied to any of the
   rays produced when cathode rays strike upon surface of a
   solid (as the wall of the vacuum tube), but now it refers
   specifically to electromagnetic radiation having wavelengths
   from 10^-3 nm to 10 nm, immediately below ultraviolet
   radiation on the wavelength scale. R["o]ntgen rays are noted
   for their penetration of 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 are one of the forms of
   ionizing radiation, which can have damaging effects on living
   cells. They also ionize gases, but cannot be reflected, or
   polarized, or deflected by a magnetic field. They are used in
   examining opaque objects, especially in medicine for
   visualizing organs and other objects inside the human body,
   as for locating fractures or bullets, and examining internal
   organs for abnormalities.
   [Webster 1913 Suppl. +PJC]

The Collaborative International Dictionary of English v.0.48:

X ray \X ray\, X-ray \X-ray\([e^]ks"r[=a]`), n. [so called by
   its discoverer because of its enigmatical character, x being
   an algebraic symbol for an unknown quantity.] (Physics)
   Originally, any of the rays produced when cathode rays strike
   upon surface of a solid (as a copper target or the wall of
   the vacuum tube); now defined as electromagnetic radiation
   with a wavelength of 0.1 to 10 nanometers. X-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, but were also referred to for
   some time as Roentgen rays. The term X-ray has become the
   most common designation. They also ionize gases, but cannot
   be reflected, or polarized, or deflected by a magnetic field.
   They are used in examining objects opaque to visible light,
   as for imaging bones or other structures inside the human
   body, and for detecting flaws in metal objects, such as in
   welds.
   [Webster 1913 Suppl. +PJC] X ray