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]

The Elements (07Nov00):

neptunium
Symbol: Np
Atomic number: 93
Atomic weight: (237)
Radioactive metallic transuranic element, belongs to the actinoids.
Np-237, the most stable isotope, has a half-life of 2.2*10^6 years and
is
a by product of nuclear reactors. The other known isotopes have mass
numbers 229 through 236, and 238 through 241. Np-236 has a half-life of
5*10^3 years. First produced by Edwin M. McMillan and P.H. Abelson in
1940.