Nepˈtūnist noun

A believer in the origin of rocks generally as chemical precipitates from the sea, opp to Plutonist or Vulcanist

neptūˈnium noun

An artificially produced radioactive transuranic metallic element (symbol Np; atomic no 93) named as next after uranium, as Neptune is next after Uranus

neptunium series noun

A series formed by the decay of artificial radio-elements, the first member being plutonium-241 and the last bismuth-209, neptunium-237 being the longest-lived