The Collaborative International Dictionary of English v.0.48:

Neptunium \Nep*tu"ni*um\, n. [NL.]
   A radioactive metallic element of atomic number 93, produced
   in nuclear reactors from Plutonium or Uranium. Symbol Np; The
   atomic weight of the most stable isotope is 237.0482.
   [PJC]

WordNet (r) 3.0 (2006):

neptunium
    n 1: a radioactive transuranic metallic element; found in trace
         amounts in uranium ores; a by-product of the production of
         plutonium [syn: neptunium, Np, atomic number 93]