nep·tu·ni·um 
(nĕp-tnē-əm, -ty-)
(nĕp-tnē-əm, -ty-)n. 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 Periodic Table.
[After the planet NEPTUNE (from the fact that it follows uranium in the periodic table).]