The Collaborative International Dictionary of English v.0.48:

Rontgen \R["o]nt"gen\, Roentgen \Roent"gen\, a.
   Of or pertaining to the German physicist Wilhelm Konrad
   R["o]ntgen, or the rays discovered by him; as, R["o]ntgen
   apparatus.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Rontgen
    n 1: German physicist who discovered x-rays and developed
         roentgenography (1845-1923) [syn: Roentgen, Wilhelm
         Konrad Roentgen, Rontgen, Wilhelm Konrad Rontgen]