- 1 Roman Mythology the god of water and of the sea. Greek equivalent Poseidon.[from Latin Neptunus]
- 2 Astronomy a distant planet of the solar system, eighth in order from the sun, discovered in 1846.
Neptune orbits between Uranus and Pluto at an average distance of 2,794 million miles (4,497 million km) from the sun (but temporarily outside the orbit of Pluto 1979–99). It is the fourth largest planet, with an equatorial diameter of 30,200 miles (48,600 km), and the most remote of the gas giants. The planet is predominantly blue, with an upper atmosphere mainly of hydrogen and helium with some methane. It has at least eight satellites, the largest of which is Triton, and a faint ring system