The Trident of Neptune was a magical Atlantean trident.
As its name implies, the trident once belonged to the God Neptune himself; in the ancient days of Atlantis, it was given to the royal family as proof of their right to rule. In the present, it's still considered a symbol of Neptune's favour, anybody who wields it can claim the right to the throne of Atlantis.
The trident itself had become lost in legend, remaining inside the ruins of the Temple of Neptune for millennia. It was recovered by Noram who planned to wield it to take over Atlantis by force.
Superman confronted Noram by using the Sword of El, which broke off one of the trident's points. After the crisis was resolved, the trident's remains were given to Superman to keep in his Fortress of Solitude for safety.
The trident is said to be imbued with the power of Neptune himself, as it was used as a focus for his command over the seas, a great deal of its master's own power swept into it. This greatly enhanced Noram's natura hydrokinetic abilities when he wielded it, making him capable of generating huge tidal waves.