Definition of Thorium 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 Noun Synonyms for word "thorium" Semanticaly linked words with "thorium" Hyponims for word "thorium" Lexical Analysis for "thorium" thoriate impregnate with thorium oxide to increase thermionic emission etymology nonthorium etymology organothorium etymology radiothorium etymology thoriferous etymology torium etymological origin of Thor etymologically related Thor is derived from eka-thorium is derived from thoria is derived from thorian is derived from thoriated is derived from thoric is derived from thorinic is derived from thorium chloride is derived from thorium dioxide is derived from thorium lead is derived from thorium nitrate is derived from thorium oxide is derived from thorium series is derived from thorium sulfate is derived from thorium sulphate is derived from thorium-230 dating is derived from thoron is derived from uranium-thorium dating