a silver-colored, radioactive, metallic chemical element, one of the actinides, produced by bombarding uranium atoms with neutrons: symbol, Np; at. no., 93
See neptunium in American Heritage Dictionary 4
(nĕp-to͞oˈnē-əm, -tyo͞oˈ-)
noun Symbol Np
A silvery, metallic, naturally radioactive element, atomic number 93, the first of the transuranium elements. Its longest-lived isotope is Np-237 with a half-life of 2.1 million years. Found in trace quantities in uranium ores, it is produced synthetically by nuclear reactions. See Table at element.