Real Dictionary

thorium in American English
a rare, grayish, radioactive, metallic chemical element, one of the actinides, found in monazite and thorite: it is used in magnesium alloys, the making of gas mantles, electronic equipment, etc., and as a nuclear fuel: symbol, Th; at. no., 90
noun: a grayish-white, lustrous, somewhat ductile and malleable, radioactive metallic element present in monazite: used as a source of nuclear energy, as a coating on sun-lamp and vacuum-tube filament coatings, and in alloys; at. wt.: 232.038; at. no.: 90; sp.

thorium in British English
noun: a soft ductile silvery-white metallic element. It is radioactive and occurs in thorite and monazite: used in gas mantles, magnesium alloys, electronic equipment, and as a nuclear power source. Symbol: Th; atomic no: 90; atomic wt: 232.0381; half-life of m