/ Dictionary / Index N neptunium: Meaning and Definition of Find definitions for: nep•tu•ni•um Pronunciation: (nep-t'nē-um, -ty'-), [key] — n. Chem., Physics. a transuranic element produced in nuclear reactors by the neutron bombardment of U-238: decays rapidly to plutonium and then to U-235. Symbol: Np; at. no.: 93. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: neptunium (Thesaurus) Neptunian neral Related Content Daily Word Quiz: dissonant Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language