Source : Webster's Revised Unabridged Dictionary (1913)

Neptunium \Nep*tu"ni*um\, n. [NL.]
   A new metallic element, of doubtful genuineness and uncertain
   indentification, said to exist in certain minerals, as
   columbite.

Source : WordNet®

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