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:

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

The Collaborative International Dictionary of English v.0.48:

X ray \X ray\, X-ray \X-ray\([e^]ks"r[=a]`), v. t.
   To examine by means of X-rays; to irradiate with X-rays.
   [PJC]

WordNet (r) 3.0 (2006):

X 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]
    2: a radiogram made by exposing photographic film to X rays;
       used in medical diagnosis [syn: roentgenogram, X ray,
       X-ray, X-ray picture, X-ray photograph]

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "X ray":
   Becquerel ray, ECG, EEG, EKG, Grenz ray, Photostat, Roentgen ray,
   X radiation, X-ray movie, Xerox, actinic ray, actinism,
   alpha radiation, alpha ray, anode ray, atomic beam, atomic ray,
   beam, beam of light, beta radiation, beta ray, blood bank,
   calotype, canal ray, capture on film, cathode ray, charity ward,
   chest X-ray, clinic, consultation room, cosmic radiation,
   cosmic ray, cosmic ray bombardment, delivery room, dispensary,
   electrocardiogram, electrocorticogram, electroencephalogram,
   electroencephalograph, electromyogram, electron emission,
   emergency, encephalogram, encephalograph, examining room,
   fever ward, film, gamma radiation, gamma ray, gleam, hospital room,
   infrared ray, infraroentgen ray, intensive care,
   invisible radiation, irradiate, isolation, labor room, laboratory,
   leam, maternity ward, microfilm, mug, nuclear rays, nursery,
   operating room, orthodiagram, pan, patch, pencil, pharmacy,
   photofluorogram, photofluorograph, photogram, photograph, photomap,
   photon, positive ray, prison ward, private room, pyelogram,
   radiation, radiogram, radiograph, radiorays, radiumize, ray,
   ray of light, recovery room, ribbon, ribbon of light, roentgenize,
   roentgenogram, roentgenograph, scotograph, semi-private room,
   shadowgram, shadowgraph, shoot, skiagram, skiagraph, snap,
   snapshoot, snapshot, solar rays, streak, stream, stream of light,
   streamer, surgery, take a photograph, talbotype, therapy,
   treatment room, ultraviolet ray, violet ray, ward