The Collaborative International Dictionary of English v.0.48:

Thorium \Tho"ri*um\, n. [NL. See Thorite.] (Chem.)
   A metallic element found in certain rare minerals, as
   thorite, pyrochlore, monazite, etc., and isolated as an
   infusible gray metallic powder which burns in the air and
   forms thoria; -- formerly called also thorinum. Symbol Th.
   Atomic weight 232.0.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thorium
    n 1: a soft silvery-white tetravalent radioactive metallic
         element; isotope 232 is used as a power source in nuclear
         reactors; occurs in thorite and in monazite sands [syn:
         thorium, Th, atomic number 90]