Source : Webster's Revised Unabridged Dictionary (1913)

Xiphura \Xi*phu"ra\, n. pl. [NL., from Gr. xi`fos sword + ?
   tail.] (Zo["o]l.)
   Same as {Limuloidea}. Called also {Xiphosura}.

   {X ray}. See under {Ray}.

. 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.

Source : WordNet®

X ray
     n 1: electromagnetic radiation of short wavelength produced when
          high-speed electrons strike a solid target [syn: {X-radiation},
           {roentgen ray}]
     2: a radiogram made by exposing photographic film to X rays;
        used in medical diagnosis [syn: {roentgenogram}, {X-ray
        picture}, {X-ray photograph}]