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