Pronunciation: (vu-nā'dē-um),[key] —n.Chem. a rare element occurring in certain minerals and obtained as a light-gray powder with a silvery luster or as a ductile metal: used as an ingredient of steel to toughen it and increase its shock resistance. Symbol: V; at. wt.: 50.942; at. no.: 23; sp. gr.: 5.96.