Neptune

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Roman Mythology The god of water, later identified with the Greek Poseidon.
• noun The sea.
• noun The eighth planet from the sun, having a sidereal period of revolution around the sun of 164.8 years at a mean distance of 4.5 billion kilometers (2.8 billion miles), a mean equatorial diameter of 49,528 kilometers (30,775 miles), and a mass 17.25 times that of Earth.

from The Century Dictionary.

• noun In Roman mythology, the god of the sea, who came to be identified by the Romans themselves with the Greek Poseidon, whose attributes were transferred by the poets to the ancient Latin deity.
• noun Figuratively, the ocean.
• noun In heraldry, same as Triton.
• noun The outermost known planet of the solar system, and the third in volume and mass, though quite invisible to the naked eye.

from the GNU version of the Collaborative International Dictionary of English.

• proper noun (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.
• proper noun (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 1026 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.
• proper noun an explosive containing nitroglycerin, -- used in blasting.
• proper noun (Zoöl.) a very large, cup-shaped, marine sponge (Thalassema Neptuni).

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun Roman mythology The god of the ocean and of earthquakes.
• proper noun The eighth planet in our solar system, represented in astronomy and astrology by ♆.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a giant planet with a ring of ice particles; the 8th planet from the sun is the most remote of the gas giants
• noun (Roman mythology) god of the sea; counterpart of Greek Poseidon