Neptune an old Italian water-deity, invested with the mythology of the Greek god of the seas and waters Poseidon, the brother of Jupiter. Poseidon was important in Homer, being hostile to the Trojans and willing to intervene on the side of the Greeks. Pallas the daughter of Jupiter, from whose head she sprang adult and fully armed. She was the patron deity of war and of many arts and crafts (see also Minerva). The original meaning of title Pallas is lost, and various myths were invented in ancient times to explain it.