Pronunciation: (thôr'ē-um, thōr'-),[key] —n.Chem. 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. Symbol: Th; at. wt.: 232.038; at. no.: 90; sp. gr.: 11.7. Cf. thoria.