Source : Webster's Revised Unabridged Dictionary (1913)

Radiothorium \Ra`di*o*tho"ri*um\, n. (Chem.)
   A radioactive substance apparently formed as a product from
   thorium.

Source : WordNet®

radiothorium
     n : radioactive isotope of thorium with mass number 228 [syn: {thorium-228}]