The Collaborative International Dictionary of English v.0.48:

Neptune \Nep"tune\, prop. n. [L. Neptunus.]
   1. (Rom. Myth.) The son of Saturn and Ops, the god of the
      waters, especially of the sea. He is represented as
      bearing a trident for a scepter.
      [1913 Webster]

   2. (Astron.) The remotest major planet of our solar system,
      discovered -- as a result of the computations of
      Leverrier, of Paris -- by Galle, of Berlin, September 23,
      1846. It is classed as a gas giant, and has a radius of
      22,716 km and an estimated mass of 1.027 x 10^26 kg,
      with an average density of 2.27 g/cc. Its mean distance
      from the sun is about 5,000,000,000 km (3,106,856,000
      miles), and its period of revolution is about 164.78
      years.
      [1913 Webster +PJC]

   Neptune powder, an explosive containing nitroglycerin, --
      used in blasting.

   Neptune's cup (Zool.), a very large, cup-shaped, marine
      sponge (Thalassema Neptuni).
      [1913 Webster]