The Collaborative International Dictionary of English v.0.48:

Ray \Ray\, n. [OF. rai, F. rais, fr. L. radius a beam or ray,
   staff, rod, spoke of a wheel. Cf. Radius.]
   1. One of a number of lines or parts diverging from a common
      point or center, like the radii of a circle; as, a star of
      six rays.
      [1913 Webster]

   2. (Bot.) A radiating part of a flower or plant; the marginal
      florets of a compound flower, as an aster or a sunflower;
      one of the pedicels of an umbel or other circular flower
      cluster; radius. See Radius.
      [1913 Webster]

   3. (Zool.)
      (a) One of the radiating spines, or cartilages, supporting
          the fins of fishes.
      (b) One of the spheromeres of a radiate, especially one of
          the arms of a starfish or an ophiuran.
          [1913 Webster]

   4. (Physics)
      (a) A line of light or heat proceeding from a radiant or
          reflecting point; a single element of light or heat
          propagated continuously; as, a solar ray; a polarized
          ray.
      (b) One of the component elements of the total radiation
          from a body; any definite or limited portion of the
          spectrum; as, the red ray; the violet ray. See Illust.
          under Light.
          [1913 Webster]

   5. Sight; perception; vision; -- from an old theory of
      vision, that sight was something which proceeded from the
      eye to the object seen.
      [1913 Webster]

            All eyes direct their rays
            On him, and crowds turn coxcombs as they gaze.
                                                  --Pope.
      [1913 Webster]

   6. (Geom.) One of a system of diverging lines passing through
      a point, and regarded as extending indefinitely in both
      directions. See Half-ray.
      [1913 Webster]

   Bundle of rays. (Geom.) See Pencil of rays, below.

   Extraordinary ray (Opt.), that one of two parts of a ray
      divided by double refraction which does not follow the
      ordinary law of refraction.

   Ordinary ray (Opt.) that one of the two parts of a ray
      divided by double refraction which follows the usual or
      ordinary law of refraction.

   Pencil of rays (Geom.), a definite system of rays.

   Ray flower, or Ray floret (Bot.), one of the marginal
      flowers of the capitulum in such composite plants as the
      aster, goldenrod, daisy, and sunflower. They have an
      elongated, strap-shaped corolla, while the corollas of the
      disk flowers are tubular and five-lobed.

   Ray point (Geom.), the common point of a pencil of rays.

   Roentgen ray, R["o]ntgen ray (r[~e]nt"g[e^]n r[=a]`)
      (Phys.), a form of electromagnetic radiation generated in
      a very highly exhausted vacuum tube by an electrical
      discharge; now more commonly called X-ray. It is
      composed of electromagnetic radiation of wavelength
      shorter than that of ultraviolet light but longer than
      that of gamma rays. It is capable of passing through many
      bodies opaque to light, and producing photographic and
      fluorescent effects by which means pictures showing the
      internal structure of opaque objects are made, called
      X-rays, radiographs, sciagraphs, X-ray photographs,
      radiograms. So called from the discoverer, W. C.
      R["o]ntgen.

   X ray, the R["o]ntgen ray; -- so called by its discoverer
      because of its enigmatical character, x being an algebraic
      symbol for an unknown quantity.
      [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Roentgen ray \Roentgen ray\
   the Anglicised spelling of R["o]ntgen ray. Same as X ray.
   [PJC]

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]

WordNet (r) 3.0 (2006):

roentgen ray
    n 1: electromagnetic radiation of short wavelength produced when
         high-speed electrons strike a solid target [syn: X ray,
         X-ray, X-radiation, roentgen ray]