uranium

ura·ni·um

noun \yu̇-ˈrā-nē-əm\

Definition of URANIUM

: a silvery heavy radioactive polyvalent metallic element that is found especially in pitchblende and uraninite and exists naturally as a mixture of three isotopes of mass number 234, 235, and 238 in the proportions of 0.006 percent, 0.71 percent, and 99.28 percent respectively—symbol U; see element table