1. In Roman Mythology, Neptune, or Neptunus, was the God of the Oceans. In Greece, he was known as Poseidon. See 'Poseidon' for more information.
2. In astronomy, Neptune is the name of one of the outer planets of our solar system, the eigth of the planets. Just like Jupiter, Saturn and Uranus, it is a gas giant. Neptune was only discovered in 1846.