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):

roentgen
    n 1: a unit of radiation exposure; the dose of ionizing
         radiation that will produce 1 electrostatic unit of
         electricity in 1 cc of dry air [syn: roentgen, R]
    2: German physicist who discovered x-rays and developed
       roentgenography (1845-1923) [syn: Roentgen, Wilhelm Konrad
       Roentgen, Rontgen, Wilhelm Konrad Rontgen]