thorium thorite • thorn · · · Info · Definitions · Synonyms · Characters · · thorium · · Text Latin · · Script 7 · · Length Definitions · · · (noun) 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. Synonyms: Th, atomic number 90. · · Noun Definition 1 Synonyms · · · th · · Synonym 1 atomic number 90 · · Synonym 2 Characters · · h latin small letter h U+0068 i latin small letter i U+0069 m latin small letter m U+006D o latin small letter o U+006F r latin small letter r U+0072 t latin small letter t U+0074 u latin small letter u U+0075