uranium
Cross references: 1. element ura.ni.um \yu.-'ra--ne--*m\ n [NL, fr. Uranus] often attrib : a silvery heavy radioactive polyvalent metallic element that is found es p. 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