Roman equivalent of Poseidon. Familiar though his name is, its origin is really very uncertain. One first seeks to link it somehow with water or the sea, and indeed attempts have been made to connect it with Greek nao, 'to flow' and naio, 'to dwell' (i.e. in the sea), and Latin nato, 'to swim' and nauta (earlier navita), 'sailor'. It may perhaps derive from some word such as Avestic (Zend) napta, 'damp' that links up with modern 'naphtha' (which is after all found in the sea).
Leonard Ashley suggests a connection with Nephele, 'cloud'. There have even been attempts to associate the name with 'potent' and 'despot', since Neptune was an 'earth-shaker'. The name of the planet Neptune, however, is as well-documented as that of the sea god is vague.
The planet was discovered in 1846 as the result of mathematical calculations by the French astronomer Urbain-Jean-Joseph Le Verrier, and he himself proposed the name in a letter to the German astronomer Johann Galle suggesting he should search for the planet. (Galle observed it five days later.)
The choice was a good one: the genealogical line Jupiter-Saturn-Uranus used for the previous three planets could not be continued since Uranus had no father. It was therefore logical to return to the brothers of Jupiter (Zeus). Thus the name of the god of the sea became that of the eighth planet of the Solar System.