What does "neptune" mean?

Neptune: (Astron.) The remotest known planet of our system, discovered -- as a result of the computations of Leverrier, of Paris -- by Galle, of Berlin, September 23, 1846. Its mean distance from the sun is about 2,775,000,000 miles, and its period of revolution is about 164,78 years. Neptune powder, an explosive containing nitroglycerin, -- used in blasting. -- Neptune's cup (Zoöl.), a very large, cup-shaped, marine sponge (Thalassema Neptuni).